Makefile.am (thread_builddir): Remove (map all existing uses to target_builddir).

Makefile.am (thread_builddir): Remove (map all existing uses to target_builddir).

	libstdc++/5697
	* include/Makefile.am (thread_builddir): Remove (map all
	existing uses to target_builddir).
	(thread_headers): Rename to...
	(thread_target_headers): ...this.
	(stamp-thread): Remove.
	(stamp-target): Correctly state the position as built.
	Update all dependencies to match the new reality.  Remove all
	dependency calculations on directories.
	* include/Makefile.in: Rebuilt.

From-SVN: r49846