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

[Jonathan Wakely]

On 10/03/17 12:12 +0000, Richard Sandiford wrote:
>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.)

Huh, I wonder why I only did it for one of them.

>Tested on arch64-linux-gnu.  OK for trunk and 6.x?

OK, thanks.