Add NuGet support for macOS

[eXpl0it3r]

The current NuGet packages don't support macOS.

I've spent quite some time getting this working, but have been unsuccessful with the RPATH/in-package dependency resolving.
This issue is a placeholder to keep track of this task.

See also #158

CSFML.2.5.2.nupkg.zip

[eXpl0it3r]

This should be a good resource for figuring this out: https://gitlab.kitware.com/cmake/community/-/wikis/doc/cmake/RPATH-handling

[eXpl0it3r]

For some reason I totally overlooked #151 which also has some hints for building on macOS

[eXpl0it3r]

I finally understand the parts of the issue and thus probably the missing puzzle pieces:

• Because we're using generic CSFML module names (e.g. csfml-graphics vs libcsfml-graphics.dylib), the search path doesn't include the .dylib suffix, despite the MS docs claiming to add those automatically.
• The library search doesn't look inside the nuget package location and/or the dylibs aren't automatically copied to the output directory, so even of they are named correctly, they are not found.
• The SFML dependencies (e.g. freetype as framework) aren't packaged with the nuget file and this can't be found.

Potential solutions:

• Add an name resolution override function that correctly handles the naming per platform.
• Look into the mechanics of how and when native library files are copied. Do they just need to match in name?
• Consider linking SFML statically and include OpenAL32 in the nuget package

[eXpl0it3r]

Well it took a lot of tinkering and adding some not so nice workarounds with rpath resolving, but it's finally working! 🥳

Please give the newly published package a try: https://www.nuget.org/packages/CSFML/2.5.2

[eXpl0it3r]

Fixed with #185