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.
