WIP: move abi checks to a feature and build the C code at build time

[pbor]

The abi tests are now conditional to the abi-tests feature,
when that is enabled, the C code for the abi tests is now built
at compile time, using the cc crate rather than having our own
Compiler abstraction and having to compile stuff at runtime and
deal with temp directories etc.

[sdroege]

Seems like a good idea

[pbor]

This currently fails for gdk-pixbuf due to gdesmott/system-deps#28

With regard to Windows CI, the good news is that this approach works and the C programs are compiled and the layout test work, the bad news is the MSVC does not support _Generic so the current code in constant.c does not work... I am not sure if we have a better idea about that (use C++?) or simply skip that test on windows

[sdroege]

It's available since Visual Studio 2019 version 16.7.0 from what I can see, and you can enable it via /std:c11. Should maybe just require that version for running the tests? What version is available with GitHub actions?

[pbor]

It's available since Visual Studio 2019 version 16.7.0 from what I can see, and you can enable it via /std:c11. Should maybe just require that version for running the tests? What version is available with GitHub actions?

Actually it seems that the Visual Studio version is new enough and does have _Generic, but however it does not like how we use it:

type 'char' in _Generic association compatible with previous association type 'char'

[sdroege]

That means we have char in there twice, which we probably shouldn't and it's gcc's fault to not complain about that

[pbor]

the problem is that we have:

        char: "%c", \
        signed char: "%hhd", \
        unsigned char: "%hhu", \

so we are printing them differently

[sdroege]

Ah that's annoying and otherwise needs conditional compilation for platforms where signed or unsigned chars are the default...

[sdroege]

OTOH, only the two cases (signed / unsigned) should be there. char alone is not really needed

[pbor]

That has helped, but now it complains about not finding <stdalign.h>, which as far as I understand should be there in MSVC when using c11. Meh.