configure.in: Update boehm-gc include dir for new GC version.

configure.in: Update boehm-gc include dir for new GC version.

	* configure.in: Update boehm-gc include dir for new GC version.
	* configure: Rebuilt.
	* exception.cc: Only include <gc.h>. Remove TRUE/FALSE hacks and
	extern "C" wrapper.
	* boehm.cc: Update includes for new GC version. MAKE_PROC is now
	GC_MAKE_PROC. mark_proc is now GC_mark_proc.
	* posix-threads.cc: Only include <gc.h>. Don't need to wrap with
	extern "C".

From-SVN: r42380