macOS/tools/contribs: support out of source build
Although contribs tarballs are placed in git repo, it doesn't interfere
with meson. The implementation is similar to vlc's bootstrap/configure.
TOOLS_BUILDDIR and CONTRIBS_BUILDDIR are relative to the caller pwd
.
Example invocation in vlcbuild
directory:
TOOLS_BUILDDIR=./keep_tools/ CONTRIBS_BUILDDIR=./keep_contribs ../vlc/extras/package/macosx/build.sh
meson doesn't support having dependencies installed in the source dir. This eases supporting contribs and vlc with meson. And later support packaging the app using meson too.