Don't test fpclassify, it's always a macro per POSIX.

Hi,

per POSIX, fpclassify is a macro:
http://pubs.opengroup.org/onlinepubs/9699919799/functions/fpclassify.html
Also, the usage of fpclassify in pl-arith.c and pl-write.c already assumes
that fpclassify is a macro as well.  The below patch just removes the test
for fpclassify from configure.in.

Corinna