These have been build options since the waf build, but that doesn't
really make sense. The build can detect whatever macOS sdk version is
available and then use that information to determine whether to enable
the features or not. Potentially disabling multiple sdk versions doesn't
really make any sense. Because f5ca11e12b
effectively made macOS 10.15 the minimum supported version, we can drop
all of these checks and bump the required sdk version to 10.15. The rest
of the build simplifies from there.
This only existed as essentially a workaround for meson's behavior and
to maintain compatibility with the waf build. Since waf put everything
in a generated subdirectory, we had to put make a subdirectory called
"generated" in the source for meson so stuff could go to the right
place. Well now we don't need to do that anymore. Move the meson.build
files around so they go in the appropriate place in the subdirectory of
the source tree and change the paths of the headers accordingly. A
couple of important things to note.
1. mpv.com now gets made in build/player/mpv.com (necessary because of
a meson limitation)
2. The macos icon generation path is shortened to
TOOLS/osxbundle/icon.icns.inc.
