This reverts commit 151abb8b.

Unfortunately has the side effect that it will prevent GCC from
generating makefile dependencies for headers from bundled external
libraries, which breaks incremental builds horribly.
(Retain the uses in configure for real system headers).

Conflicts:
	RepositoryExternal.mk
	configure.ac

Change-Id: I149db1d402fa18bdc470f90dee846cfb5158237e