Use -std=gnu11 instead of -std=gnu99.

Now that GCC 4.7 or later is required to build glibc, this patch moves
the build from using -std=gnu99 to -std=gnu11 (option added in 4.7).
This allows use of C11 features from GCC's headers, such as new
float.h macros and max_align_t.

Tested for x86_64 and x86 (testsuite; installed stripped shared
libraries are unchanged by the patch on x86_64, while I see some
slight code reordering of no significance on x86).

	* Makeconfig (CFLAGS): Use -std=gnu11 instead of -std=gnu99.
	* Makefile ($(objpfx)c++-types-check.out): Filter out -std=gnu11
	instead of -std=gnu99.
	* configure.ac (systemtap): Test with -std=gnu11 instead of
	-std=gnu99.
	* configure: Regenerated.
	* math/gen-auto-libm-tests.c: Use -std=gnu11 instead of -std=gnu99
	in compilation command in comment.