Convert libGL and DRI drivers to require libdrm.

libdrm can be had from:

http://people.freedesktop.org/~ajax/libdrm/
diff --git a/configs/default b/configs/default
index edc7618..45efc81 100644
--- a/configs/default
+++ b/configs/default
@@ -10,7 +10,7 @@
 MESA_MINOR=3
 MESA_TINY=2
 
-# external projects
+# external projects.  This should be useless now that we use libdrm.
 DRM_SOURCE_PATH=$(TOP)/../drm
 
 # Compiler and flags