Remove -I and -L flags from CPPFLAGS, CFLAGS, CXXFLAGS, OBJCFLAGS, LDFLAGS.

Note there are two significant commits here:

Fetch jhbuildrc-gtk-osx from xpra fork. This is necessary for any local changes to jhbuildrc-gtk-osx to take effect. Note that the local fork of jhbuildrc-gtk-osx has drifted from upstream: https://paste.debian.net/1310060/ The diff seems benign. See this CI run which succeeded with the local fork: https://github.com/cpatulea/gtk-osx-build/actions/runs/8151698638
Remove -I and -L flags from CPPFLAGS, CFLAGS, CXXFLAGS, OBJCFLAGS, LDFLAGS. This is the main change. It should fix the build of libvpx 1.14.0 (see discussion: https://github.com/Xpra-org/gtk-osx-build/commit/0a1a8d83ef52f08c3266baeb9ed7f1d8dc6ee761). Same CI run as above ran with this change and shows the build succeeds with this change. Upstream gtk-osx-build may look at this change and consider merging it in ~April 2024.

Xpra-org / gtk-osx-build