Skip -std for C source files

[alandefreitas]

MrDocs appends -std=c++23 and uses a bundled version of libcxx so we get reproducible results. But we don't check whether the file is cpp or c.

error: invalid argument '-std=c++23' not allowed with 'C'
In file included from /home/ruben/workspace/boost-root/libs/charconv/src/to_chars.cpp:7:
/home/ruben/workspace/boost-root/libs/charconv/src/float128_impl.hpp:21:10: fatal error: 'quadmath.h' file not found
   21 | #include <quadmath.h>
      |          ^~~~~~~~~~~~
In file included from /home/ruben/workspace/boost-root/libs/charconv/src/from_chars.cpp:14:
/home/ruben/workspace/boost-root/libs/charconv/src/float128_impl.hpp:21:10: fatal error: 'quadmath.h' file not found
   21 | #include <quadmath.h>
      |          ^~~~~~~~~~~~
1 error generated.
1 error generated.
warning: unsupported HTML tag <b>

An issue occurred during execution.
If you believe this is a bug, please report it at https://github.com/cppalliance/mrdocs/issues
with the following details:
    MrDocs Version: 0.0.2 (Build: )
    Reported From: `src/lib/AST/ParseJavadoc.cpp` at line 593

// Omitted