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.

test/CMakeLists.txt

@@ -242,6 +242,14 @@ if (FMT_CUDA_TEST)
 endif ()
 
 enable_language(C)
-add_executable(c-test c-test.c)
-target_link_libraries(c-test PRIVATE fmt::fmt-c)
-add_test(NAME c-test COMMAND c-test)
+# Clang < 3.8 doesn't apply array-to-pointer decay to the controlling
+# expression of _Generic (https://llvm.org/PR16340), so fmt-c.h's argument
+# dispatch doesn't match string literals there.
+if (CMAKE_C_COMPILER_ID STREQUAL "Clang" AND CMAKE_C_COMPILER_VERSION
+                                             VERSION_LESS 3.8)
+  message(STATUS "Skipping c-test: _Generic is broken in Clang < 3.8")
+else ()
+  add_executable(c-test c-test.c)
+  target_link_libraries(c-test PRIVATE fmt::fmt-c)
+  add_test(NAME c-test COMMAND c-test)
+endif ()