FindQuadmath: add -fext-numeric-literals if the compiler supports it

this is required for GCC >= 4.8 to support the 'Q' suffix for floating
point literals (which are used in the quadmath.h header)

In order to prevent compilers which eat the flag but do not support it
from complaining constantly, it is tested for with "-Werror" enabled...
Merge pull request #631 from andlaus/add_-fext-numeric-literals

FindQuadmath: add -fext-numeric-literals if the compiler supports it