Fix a compile time issue with _GNU_SOURCE alongside modern Ruby and/or GCC et al.

This is to fix the following issue:

  In file included from ../../../../ext/magic/ruby-magic.c:21:0:
  ../../../../ext/magic/ruby-magic.h:96:8: error: redefinition of 'struct exception'
  /usr/include/math.h:321:8: note: originally defined here
  make: *** [ruby-magic.o] Error 1

Signed-off-by: Krzysztof Wilczynski <krzysztof.wilczynski@linux.com>