Set matching CC alongside CXX in Linux CI matrix (#4863)

* Set matching CC alongside CXX in Linux CI matrix

The Configure step only set CXX, so CMake's C compiler detection
fell back to whatever the default happened to be on the runner,
independent of which C++ compiler the matrix entry was actually
testing (e.g. CXX=clang++-3.6 but CC left to detect GCC 11).

Derive CC from the same matrix.cxx value the job already installs
a matching compiler for.

* Skip c-test when the C compiler is Clang < 3.8

Setting CC from matrix.cxx means the clang++-3.6 job now configures with real
clang 3.6 instead of falling back to the runner's default GCC, which exposes a
compile failure in test/c-test.c: clang predates the LLVM PR16340 fix (landed in
3.8) that applies array-to-pointer decay to the controlling expression of
_Generic, so fmt-c.h's FMT_MAKE_ARG dispatch never matches a string literal
(char[4]) against its char*/const char* associations. It falls through to the
zero-argument default association, which is then called with one argument.

The guard is at configure time rather than in the CI Test step. c-test is part
of the default target, so `cmake --build` compiles it before ctest ever runs --
excluding it with `ctest -E` cannot help, because the job has already failed in
the Build step. Guarding the add_executable also means anyone building fmt's
tests with an old clang benefits, not just this one CI job.

enable_language(C) stays first and unconditional, since CMAKE_C_COMPILER_VERSION
is not set before it. fmt-c itself (src/fmt-c.cc, C++) still builds on that job;
only the C-consumer smoke test is skipped.

.github/workflows/linux.yml

@@ -179,6 +179,13 @@ jobs:
         sudo apt install locales-all
         cmake -E make_directory ${{runner.workspace}}/build
 
+    - name: Select matching C compiler
+      run: |
+        cc=${{matrix.cxx}}
+        cc=${cc/clang++/clang}
+        cc=${cc/g++/gcc}
+        echo "CC=$cc" >> "$GITHUB_ENV"
+
     - name: Configure
       working-directory: ${{runner.workspace}}/build
       env: