Add Win32 feature test macros

Checking for the UCRT is not quite straightforward, as one has to include vcruntime.h on MSVC (present only since MSVC 2015), or a CRT header on mingw-w64 toolchains. Moreover, it makes sense to do some configuration testing in a central place; as such we introduce (private) feature macros for the UCRT (among other things..)

I'm going to open other MR following this that will make use of the introduced macros