Make copy of <bits/std_abs.h> from GCC 7 [BZ #21573]

Make copy of <bits/std_abs.h> from GCC 7 [BZ #21573]

<bits/std_abs.h> from GCC 7 will include /usr/include/stdlib.h from
"#include_next" (instead of stdlib/stdlib.h in the glibc source
directory), and this turns up as a make dependency.  Also make a copy
of <bits/std_abs.h> to prevent it from including /usr/include/stdlib.h.

	[BZ #21573]
	* Makerules [$(c++-bits-std_abs-h) != ""] (before-compile): Add
	$(common-objpfx)bits/std_abs.h.
	[$(c++-bits-std_abs-h) != ""] ($(common-objpfx)bits/std_abs.h):
	New target.
	* config.make.in (c++-bits-std_abs-h): New.
	* configure.ac (find_cxx_header): Use "\,$1," with sed.
	(CXX_BITS_STD_ABS_H): New.
	(AC_SUBST(CXX_BITS_STD_ABS_H)): Likewise.
	* configure: Regenerated.