Ensure C99 and C11 interfaces are available for C++ [BZ #21326]

Ensure C99 and C11 interfaces are available for C++ [BZ #21326]

This patch ensures that the C99 and C11 features required by C++ are
defined according to the value of the __cplusplus macro, and not just
because G++ always defines _GNU_SOURCE.  This will allow G++ to stop
defining _GNU_SOURCE some day, without causing the C99 and C11
interfaces to disappear for C++ programs.

	[BZ #21326]
	* include/features.h [__cplusplus >= 201103] (__USE_ISOC99): Define.
	[__cplusplus >= 201703] (__USE_ISOCXX17, __USE_ISOC11): Define.
	* math/Makefile (test-math-cxx11): New test.
	* math/test-math-cxx11.cc: New file.