Document the required C version or compiler version

[RokerHRO]

I'd like to make some PR but would like to know in advance which C version or compiler version has to be supported, e.g. "ISO C90" or is C99 acceptable, too?

What is the oldest compiler version of e.g. gcc, clang and MSVC that must still be able to compile the code?

[bk138]

I would say c90 as nothing in the code is using c99 features yet. can we formalise this in cmakelists?

[RokerHRO]

1. Cmakelists ist okay, but I'd add a clear and easy to find statement about the C version and/or supported compiler version in the documentation or in the CONTRIBUTING.md, too. For the interested human reader. :-)

2. I ask because I'd like to write

char *s = something();

instead of

char *s;
…
s = something();