[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.

From-SVN: r246025

libstdc++-v3/ChangeLog

@@ -1,3 +1,9 @@
+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.
+
 2017-03-09  Jonathan Wakely  <jwakely@redhat.com>
 
 	* include/std/functional (_Not_fn): Define macro to simplify

libstdc++-v3/acinclude.m4

@@ -2297,7 +2297,8 @@ AC_DEFUN([GLIBCXX_CHECK_MATH11_PROTO], [
       AC_MSG_CHECKING([for obsolete isnan function in <math.h>])
         AC_CACHE_VAL(glibcxx_cv_obsolete_isnan, [
           AC_COMPILE_IFELSE([AC_LANG_SOURCE(
-            [#include <math.h>
+            [#define _GLIBCXX_INCLUDE_NEXT_C_HEADERS
+             #include <math.h>
              #undef isnan
              namespace std {
                using ::isnan;

libstdc++-v3/configure

@@ -18390,6 +18390,7 @@ else
 
           cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
+#define _GLIBCXX_INCLUDE_NEXT_C_HEADERS
 #include <math.h>
              #undef isnan
              namespace std {