bin/install_megadrivers: fix DESTDIR and -D*-path

This fixes -Ddri-drivers-path, -Dvdpau-libs-path, etc. with DESTDIR when
those paths are absolute. Currently due to the way python's os.path.join
handles absolute paths these will ignore DESTDIR, which is bad. This
fixes them to be relative to DESTDIR if that is set.

Fixes: 3218056e0e
       ("meson: Build i965 and dri stack")
Signed-off-by: Dylan Baker <dylan.c.baker@intel.com>

bin/install_megadrivers.py

@@ -1,6 +1,6 @@
 #!/usr/bin/env python
 # encoding=utf-8
-# Copyright © 2017 Intel Corporation
+# Copyright © 2017-2018 Intel Corporation
 
 # Permission is hereby granted, free of charge, to any person obtaining a copy
 # of this software and associated documentation files (the "Software"), to deal
@@ -35,7 +35,11 @@ def main():
     parser.add_argument('drivers', nargs='+')
     args = parser.parse_args()
 
-    to = os.path.join(os.environ.get('MESON_INSTALL_DESTDIR_PREFIX'), args.libdir)
+    if os.path.isabs(args.libdir):
+        to = os.path.join(os.environ.get('DESTDIR', '/'), args.libdir[1:])
+    else:
+        to = os.path.join(os.environ['MESON_INSTALL_DESTDIR_PREFIX'], args.libdir)
+
     master = os.path.join(to, os.path.basename(args.megadriver))
 
     if not os.path.exists(to):