graphics/graphene: fix build on GCC architectures

Base GCC doesn't like this:
/usr/include/math.h: In function 'inline_isnan':
/usr/include/math.h:200: error: comparing floating point with == or != is unsafe
/usr/include/math.h: In function 'inline_isnanf':
/usr/include/math.h:207: error: comparing floating point with == or != is unsafe
/usr/include/math.h: In function '__inline_isnanl':
/usr/include/math.h:214: error: comparing floating point with == or != is unsafe