gdb: Build on Darwin

Note that it doesn't actually work unless you run it as root, because
the Darwin kernel disallows unsigned debuggers (you'll get an error
message "please check gdb is codesigned").
qt5: disable parallel building, as it often fails
mesa cleanup: remove patches, clean description

werror-wundef.patch came from #1082, but now no longer applied,
and it seems no longer necessary, as both qt5 and qtCreator build.
@bbenoist: was the patch useful for anything else?
Merge master into x-updates (to fix qt5 build)