CI: Check compiler warnings.

Add checks to make CI fail by compiler warnings in the `rake compile`.

If the `skip-warn-checks` (default: `false`) is `true` in specific matrix
cases, the cases skip the checks. If you want to skip new compiler warnings
coming from external changes such as upgraded compiler or OpenSSL versions in
the specific matrix cases, you can set the `skip-warn-checks: true` for the
cases.

.github/workflows/test.yml

@@ -19,6 +19,7 @@ jobs:
         # ubuntu-22.04 uses OpenSSL 3.0, ubuntu-20.04 uses OpenSSL 1.1.1
         os: [ ubuntu-22.04, ubuntu-20.04, macos-latest, windows-latest ]
         ruby: ${{ fromJson(needs.ruby-versions.outputs.versions) }}
+        skip-warn-checks: [ false ]
         exclude:
           # uses non-standard MSYS2 OpenSSL 3 package
           - { os: windows-latest, ruby: head }
@@ -47,6 +48,10 @@ jobs:
         run:  echo "MAKEFLAGS=V=1" >> $GITHUB_ENV
         if: runner.os == 'Linux' || runner.os == 'macOS'
 
+      - name: set flags to check compiler warnings.
+        run:  echo "RUBY_OPENSSL_EXTCFLAGS=-Werror" >> $GITHUB_ENV
+        if: ${{ !matrix.skip-warn-checks }}
+
       - name: compile
         run:  rake compile -- --enable-debug
 
@@ -79,6 +84,7 @@ jobs:
           - libressl-3.6.3
           - libressl-3.7.3
           - libressl-3.8.0 # Development release
+        skip-warn-checks: [ false ]
         fips-enabled: [ false ]
         include:
           - { os: ubuntu-latest, ruby: "3.0", openssl: openssl-3.0.9, fips-enabled: true, append-configure: 'enable-fips', name-extra: 'fips' }
@@ -142,6 +148,10 @@ jobs:
         run:  echo "MAKEFLAGS=V=1" >> $GITHUB_ENV
         if: runner.os == 'Linux' || runner.os == 'macOS'
 
+      - name: set flags to check compiler warnings.
+        run:  echo "RUBY_OPENSSL_EXTCFLAGS=-Werror" >> $GITHUB_ENV
+        if: ${{ !matrix.skip-warn-checks }}
+
       - name: compile
         run:  rake compile -- --enable-debug --with-openssl-dir=$HOME/.openssl/${{ matrix.openssl }}