hw/xwin: Look for gl spec files in $(srcdir) or .
Tarballs include the downloaded gl spec files, which will end up in $(srcdir). But, git-based builds will not have them at all and will need to download them from opengl.org. They'll land in in the build directory instead of $(srcdir), and so we need to allow them to be in either place. This change checks for the files in $(srcdir), linking them to . if present. Otherwise, it downloads them from opengl.org. A suggested better solution is to have Mesa install these files somewhere. Signed-off-by: Keith Packard <firstname.lastname@example.org> Tested-by: Kristian Høgsberg <email@example.com>