Configure feature test macros in meson

[rom1v]

Use add_global_arguments() like in PR #2807 add_project_arguments().

However, the documentation says:

Compilation tests that are run as part of your project configuration do not use these flags. The reason for that is that you may need to run a test compile with and without a given flag to determine your build setup. For this reason tests do not use these global arguments.

We could like these definitions to be set in tests too. But in practice, they are correctly set on my machine 🤔

An alternative could be to add the flags (still from Meson) to the config.h generated file. I don't know what is better. Any idea?

@mqiezi Could you confirm that this does not cause a regression for #1960?

cc @RipleyTom

[RipleyTom]

Yes add_project_arguments seems to be the appropriate one, sorry I never edited a meson build before.
Shouldn't the _WIN32_WINNT and WINVER also be defined when crosscompiling?

[rom1v]

Shouldn't the _WIN32_WINNT and WINVER also be defined when crosscompiling?

host_machine == "windows" when cross-compiling:

The tl/dr summary is the following: if you are doing regular cross compilation, you only care about build_machine and host_machine. Just ignore target_machine altogether and you will be correct 99% of the time.

[rom1v]

Superseded by #2813.