[build] don't try to create src/GLdispatch/mapi makefiles

The build doesn't recurse into these directories, so their Makefile.in files
aren't propagated to the dist tarball.  That makes configure fail because it
tries to read those files to generate Makefiles in those directories.

Since these directories aren't built, don't try to create Makefiles in them.

Signed-off-by: Aaron Plattner <aplattner@nvidia.com>
Signed-off-by: Brian Nguyen <brnguyen@nvidia.com>