[build] Shift __cplusplus check to a higher level for bool type define

[lamweilun]

The main reason for this change is that, C++ compilers, or at least g++ and clang++ from my own testing, defines STDC but not STDC_VERSION. And when compiling with -Wundef, it would trigger an error when its checking for STDC_VERSION.

This change basically says, check if you're a C++ compiler first before even entering any of the bool type definition since C++ has bool as a built-in type.

[raysan5]

@lamweilun Please, could you split those changes into two separate PRs, also, raymath convention is avoiding functions calling other functions, please, review that, thanks!

[lamweilun]

@raysan5 Thanks for the review. I have kept this to just the shifting of the C++ check

[raysan5]

@lamweilun thanks for the review... wondering why it was not reported before...