Support --disable-fixincludes.

Always install limits.h and syslimits.h header files
to include folder.

When --disable-fixincludes is used, then no system header files
are fixed by the tools in fixincludes. Moreover, the fixincludes
tools are not built any longer.

gcc/ChangeLog:

	* Makefile.in: Always install limits.h and syslimits.h to
	include folder.
	* configure.ac: Assign STMP_FIXINC blank if
	--disable-fixincludes is used.
	* configure: Regenerate.