Adds a github action workflow.

.github/scripts/build_and_test.sh

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+#! /bin/bash
+
+cpanm -vn Net::SSLeay
+cpanm -n Alien::Build
+cpanm Alien::GSL
+cpanm Module::Build
+mkdir -p xs
+perl Build.PL
+./Build installdeps --cpan_client cpanm
+./Build
+./Build test

.github/scripts/build_dist.sh

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+#! /bin/bash
+
+export LD_LIBRARY_PATH=${GSL_DIR}/lib
+./Build dist
+DISTNAME=$(ls -1 Math*.tar.gz)
+echo "MATH_GSL_DISTNAME=$DISTNAME" >> $GITHUB_ENV
+echo "$DISTNAME" > math-gsl-dist-name.txt

.github/scripts/install_gsl.sh

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+#! /bin/bash
+
+GSL_MIRROR=http://mirrors.kernel.org/gnu/gsl
+BUILD_DIR=$GSL_DIR/build
+INSTALL_DIR=$GSL_DIR
+mkdir -p $BUILD_DIR
+cd "$BUILD_DIR"
+export PATH=$PERL_DIR/bin:$PATH
+ZIP_NAME="$GSL_NAME".tar.gz
+wget "$GSL_MIRROR"/"$ZIP_NAME" --retry-connrefused --timeout=900
+tar zxvf "$ZIP_NAME"
+cd "$GSL_NAME"
+./configure --prefix "$INSTALL_DIR"
+make -j$(nproc)
+make install
+echo "PATH=$PATH" >> $GITHUB_ENV

.github/scripts/install_perl.sh

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+#! /bin/bash
+
+# NOTE: perl version 5.26.1 is already installed (on Ubuntu 20.04),
+#   but we would like to not mess with that, so we install a custom perl.
+
+BUILD_DIR=$PERL_DIR/build
+INSTALL_DIR=$PERL_DIR
+mkdir -p $BUILD_DIR
+cd "$BUILD_DIR"
+ZIP_NAME=${PERL_NAME}.tar.gz
+wget https://www.cpan.org/src/5.0/$ZIP_NAME
+ZIP_TYPE=$(perl -pe 's/^.*\.([^.]*)$/$1/' <<<$ZIP_NAME)
+TAR_NAME=$(perl -pe 's/^(.*)\.[^.]*$/$1/' <<<$ZIP_NAME)
+if [[ $ZIP_TYPE == 'gz' ]] ; then
+    gunzip $ZIP_NAME
+elif [[ $ZIP_TYPE == 'bz2' ]] ; then
+    bunzip2 $ZIP_NAME
+else
+    echo "Unknown archive type: $ZIP_TYPE"
+    exit 1
+fi
+tar xvf $TAR_NAME
+PERL_DIR2=$(perl -pe 's/\.tar$//' <<<$TAR_NAME)
+cd $PERL_DIR2
+sh Configure -des -Dprefix=$INSTALL_DIR -Dman1dir=none -Dman3dir=none
+make
+make install
+export PATH=$INSTALL_DIR/bin:$PATH
+echo "PATH=$PATH" >> $GITHUB_ENV
+perl --version
+cpan -v
+cpan App::cpanminus
+

.github/scripts/set_environment_variables.sh

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+#! /bin/bash
+
+if [[ -n $PERL_DIR ]] ; then
+    PERL_DIR="${PERL_DIR/#\~/$HOME}"
+    echo "PERL_NAME=$PERL_NAME" >> $GITHUB_ENV
+    echo "PERL_DIR=$PERL_DIR" >> $GITHUB_ENV
+    export PATH=$PERL_DIR/bin:"$PATH"
+fi
+if [[ -n $GSL_DIR ]] ; then
+    # autoconf needs an absolute prefix to install GSL, replace tilde with $HOME
+    GSL_DIR="${GSL_DIR/#\~/$HOME}"
+    echo "GSL_NAME=$GSL_NAME" >> $GITHUB_ENV
+    echo "GSL_DIR=$GSL_DIR" >> $GITHUB_ENV
+    export LD_LIBRARY_PATH=$GSL_DIR/lib
+    export PATH=$GSL_DIR/bin:"$PATH"
+    export PKG_CONFIG_PATH="$GSL_DIR"/lib/pkgconfig
+fi
+
+echo "LD_LIBRARY_PATH=$LD_LIBRARY_PATH" >> $GITHUB_ENV
+echo "PATH=$PATH" >> $GITHUB_ENV
+echo "PKG_CONFIG_PATH=$PKG_CONFIG_PATH" >> $GITHUB_ENV

.github/scripts/show_perl_info.sh

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+#! /bin/bash
+
+perl --version
+export PATH=$GITHUB_WORKSPACE/perl/bin:$PATH
+echo "PATH=$PATH" >> $GITHUB_ENV
+perl --version
+cpan -v
+#uname -a
+#lsb_release -d

.github/workflows/actions.yml

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+name: linux-build-dist
+on: [push, pull_request]
+jobs:
+  build-perls:
+    runs-on: ubuntu-20.04
+    strategy:
+      matrix:
+        perl: [34, 32, 30, 28, 26]
+    steps:
+      - uses: actions/checkout@v2
+      - name: set environment variables
+        env:
+          PERL_NAME: perl-5.${{ matrix.perl }}.0
+          PERL_DIR: ~/perl/${{ runner.os }}/5.${{ matrix.perl }}
+        run: |
+          ./.github/scripts/set_environment_variables.sh
+      - name: perl cache
+        env:
+          CACHE_KEY: ${{ runner.os }}-perl3-${{ matrix.perl }}
+        id: cache-perl
+        uses: actions/cache@v2
+        with:
+          path: ${{ env.PERL_DIR }}
+          key: ${{ env.CACHE_KEY }}
+      - name: install perl
+        if: steps.cache-perl.outputs.cache-hit != 'true'
+        run: |
+          ./.github/scripts/install_perl.sh
+  build-gsl:
+    runs-on: ubuntu-20.04
+    strategy:
+      matrix:
+        gsl: [2.7, 2.6, 2.5, 2.4, 2.3]
+    steps:
+      - uses: actions/checkout@v2
+      - name: set environment variables
+        env:
+          GSL_NAME: gsl-${{ matrix.gsl }}
+          GSL_DIR: ~/gsl/${{ runner.os }}/gsl-${{ matrix.gsl }}
+        run: |
+          ./.github/scripts/set_environment_variables.sh
+      - name: gsl cache
+        env:
+          CACHE_KEY: ${{ runner.os }}-gsl4-${{ matrix.gsl }}
+        id: cache-gsl
+        uses: actions/cache@v2
+        with:
+          path: ${{ env.GSL_DIR }}
+          key: ${{ env.CACHE_KEY }}
+      - name: install gsl
+        if: steps.cache-gsl.outputs.cache-hit != 'true'
+        run: |
+          ./.github/scripts/install_gsl.sh
+  build-dist:
+    runs-on: ubuntu-20.04
+    needs: [build-gsl, build-perls]
+    strategy:
+      matrix:
+        gsl: [2.7]
+        perl: [34]
+    steps:
+      - uses: actions/checkout@v2
+      - name: set environment variables
+        env:
+          GSL_NAME: gsl-${{ matrix.gsl }}
+          GSL_DIR: ~/gsl/${{ runner.os }}/gsl-${{ matrix.gsl }}
+          PERL_NAME: perl-5.${{ matrix.perl }}.0
+          PERL_DIR: ~/perl/${{ runner.os }}/5.${{ matrix.perl }}
+        run: |
+          ./.github/scripts/set_environment_variables.sh
+
+      - name: perl cache
+        env:
+          CACHE_KEY: ${{ runner.os }}-perl3-${{ matrix.perl }}
+        id: cache-perl
+        uses: actions/cache@v2
+        with:
+          path: ${{ env.PERL_DIR }}
+          key: ${{ env.CACHE_KEY }}
+      - name: gsl cache
+        env:
+          CACHE_KEY: ${{ runner.os }}-gsl4-${{ matrix.gsl }}
+        id: cache-gsl
+        uses: actions/cache@v2
+        with:
+          path: ${{ env.GSL_DIR }}
+          key: ${{ env.CACHE_KEY }}
+      - name: install packages
+        run: |
+          # NOTE: The following packages are already installed in "ubuntu-20.04" :
+          #     build-essential curl g++ git wget libssl-dev libz-dev
+          #       swig autoconf libtool
+          #sudo apt-get install swig autoconf libtool
+          swig -version
+      - name: install perl modules, build, and test
+        run: |
+          ./.github/scripts/build_and_test.sh
+      - name: build dist
+        run: |
+          ./.github/scripts/build_dist.sh
+      - name: Save dist as artifact
+        uses: actions/upload-artifact@v2
+        with:
+          name: math_gsl_dist
+          path: ${{ env.MATH_GSL_DISTNAME }}
+      - name: Save name of dist as artifact
+        uses: actions/upload-artifact@v2
+        with:
+          name: math_gsl_dist_name
+          path: math-gsl-dist-name.txt
+  install-dist-linux:
+    runs-on: ubuntu-20.04
+    needs: [build-dist]
+    strategy:
+      matrix:
+        gsl: [2.7, 2.6, 2.5, 2.4, 2.3]
+        perl: [34, 32, 30, 28, 26]
+    steps:
+      - uses: actions/checkout@v2
+      - name: set environment variables
+        env:
+          GSL_NAME: gsl-${{ matrix.gsl }}
+          GSL_DIR: ~/gsl/${{ runner.os }}/gsl-${{ matrix.gsl }}
+          PERL_NAME: perl-5.${{ matrix.perl }}.0
+          PERL_DIR: ~/perl/${{ runner.os }}/5.${{ matrix.perl }}
+        run: |
+          ./.github/scripts/set_environment_variables.sh
+      - name: perl cache
+        env:
+          CACHE_KEY: ${{ runner.os }}-perl3-${{ matrix.perl }}
+        id: cache-perl
+        uses: actions/cache@v2
+        with:
+          path: ${{ env.PERL_DIR }}
+          key: ${{ env.CACHE_KEY }}
+      - name: gsl cache
+        env:
+          CACHE_KEY: ${{ runner.os }}-gsl4-${{ matrix.gsl }}
+        id: cache-gsl
+        uses: actions/cache@v2
+        with:
+          path: ${{ env.GSL_DIR }}
+          key: ${{ env.CACHE_KEY }}
+      - name: Download dist
+        env:
+          PERL_NAME: perl-5.${{ matrix.perl }}
+          PERL_DIR: ~/perl/${{runner.os}}/5.${{ matrix.perl }}
+          GSL_NAME: gsl-${{ matrix.gsl }}
+          GSL_DIR: ~/gsl/${{runner.os}}/gsl-${{ matrix.gsl }}
+        uses: actions/download-artifact@v2
+        with:
+          name: math_gsl_dist
+      - name: Download dist name
+        uses: actions/download-artifact@v2
+        with:
+          name: math_gsl_dist_name
+      - name: extract dist
+        run: |
+          DISTNAME=$(<math-gsl-dist-name.txt)
+          tar zxvf $DISTNAME
+          DISTDIR=$(perl -pe 's/\.tar.gz$//' <<<"$DISTNAME")
+          echo "DISTDIR=$DISTDIR" >> $GITHUB_ENV
+      - name: install perl modules and build and test
+        run: |
+          BASEDIR=$PWD
+          cd $DISTDIR
+          $BASEDIR/.github/scripts/build_and_test.sh