osdep: add locale includes for APPLE and WIN32

Created by: tguillem

Tested on macOS (build + run) but not tested on windows.

Not sure if I'm fixing the meson script the right way.

Created by: tguillem

Just added the locale fix for Windows and tested the cross compilation using mingw32.

Not sure if I'm fixing the meson script the right way.

The one thing that catches my eye is that it doesn't really scale well to add msvcr100 to the build_deps like that from inside the check for useloc. Depending on other requirements we might need to add that in other cases as well. But as long as this is the only os-dependent feature in libplacebo (and I don't foresee that changing too quickly), I suppose it's fine.

The nested if/else chains are also sort of weird, but I'll change that in an independent commit. Regardless, if it works (I have no way of verifying this), I suppose the actual implementation is fine.

It would be nice to have a way to get a locale-invariant printf without requiring newlocale, which I think might be possible on windows; but I can't find good documentation for this either, so let's just leave it at that.

Merged.