mesa: Convert gl.pc to be generated by configure.

This saves a step of mashing variables around in our Makefile.
diff --git a/configure.ac b/configure.ac
index 532feaa..a5df38a 100644
--- a/configure.ac
+++ b/configure.ac
@@ -2185,7 +2185,8 @@
 		src/mesa/drivers/dri/r200/Makefile
 		src/mesa/drivers/dri/radeon/Makefile
 		src/mesa/drivers/dri/swrast/Makefile
-		src/mesa/drivers/x11/Makefile])
+		src/mesa/drivers/x11/Makefile
+		src/mesa/gl.pc])
 
 dnl Sort the dirs alphabetically
 GALLIUM_TARGET_DIRS=`echo $GALLIUM_TARGET_DIRS|tr " " "\n"|sort -u|tr "\n" " "`