CompilerId: Restore support for classic C by avoiding C++ style comments
C++ style comments were added by commit fc3a1cbd (CompilerID: Compiler
extensions default detection, 2021-05-29, v3.22.0-rc1~52^2~2), but they
may not be supported by the default mode of some C compilers. Use
C-style comments instead. For consistency, do this for all languages.
49 jobs for !6759 with compiler-id-classic-c in 12 minutes and 52 seconds
(queued for 2 seconds)