Define _GNU_SOURCE on Linux for mremap()

This was committed to CPython's libffi copy in
https://bugs.python.org/issue10309

mremap() documentation says _GNU_SOURCE needs to
be defined in order to use mremap(): see the
synopsis section at http://linux.die.net/man/2/mremap

Original commit: https://hg.python.org/cpython/rev/9986fff720a2

Original patch was written by Hallvard B Furuseth.
1 file changed