Please add C4_MINGW macro for rapidyaml 0.7.0

[toge]

Thanks for providing such a very fast libraries!
In mingw64 compiler, there are compilation errors on rapidyaml 0.7.0:

In file included from src/c4/yml/tree.hpp:12,
                 from src/c4/yml/reference_resolver.hpp:4,
                 from src/c4/yml/reference_resolver.cpp:1:
src/c4/yml/common.hpp:14:10: fatal error: alloca.h: No such file or directory
   14 | #include <alloca.h>
      |          ^~~~~~~~~~
compilation terminated.

This is due to the lack of alloca.h in the mingw compiler as well as in the MSVC compiler.
I want to add C4_MINGW macro to c4core so that rapidyaml uses malloc.h instead of alloca.h on mingw.

#if defined(C4_MSVC) || defined(C4_MINGW)
#include <malloc.h>
#else
#include <alloca.h>
#endif

This same fix is also implemented in imgui.
ocornut/imgui#276

[biojppm]

Thanks!