[libstdc++-v3] Fix detection of obsolete isnan

[libstdc++-v3] Fix detection of obsolete isnan

libstdc++-v3 configure checks whether old glibc inline definitions
of isnan would conflict with the libstdc++-v3 definitions and
works around them if so.  But if g++ 6.x build A is used to build
another g++ 6.x B, the configure step for B will pick up the math.h
installed alongside A instead of the glibc version.  configure will
then assume that the workaround isn't necessary, leaving B with a
broken cmath.

isinf already worked around this.  This patch extends the same fix
to isnan.  (Thanks to George for the fix.)

libstdc++-v3/
2017-03-10  George Lander  <george.lander@arm.com>

	* acinclude.m4 (glibcxx_cv_obsolete_isnan): Define
	_GLIBCXX_INCLUDE_NEXT_C_HEADERS before including math.h.
	* configure: Regenerate.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@246025 138bc75d-0d04-0410-961f-82ee72b054a4