feat(ci): add build forcing c++ mode

Add a distcheck build that sets CFLAGS such that it treats all files as
C++, preventing pull requests from breaking C++ compatibility going
forward.

stack-info: PR: #578, branch: aws-nslick/stack/25

.github/workflows/distcheck.yaml

@@ -95,6 +95,9 @@ jobs:
         sdk:
           - cuda
           - neuron
+        ccmode:
+          - c
+          - cpp
 
         include:
           - cc-variant: latest
@@ -182,6 +185,11 @@ jobs:
 
           export CC="${{ matrix.realcc || matrix.cc }}"
 
+          if [ "${{ matrix.ccmode }}" == "cpp" ]; then
+            export CC="${CC/clang/clang++}"
+            export CC="${CC/gcc/g++}"
+          fi
+
           # actions/checkout@v4 would drop the plugin source in $PWD,
           # so go ahead and build it.
           ./autogen.sh