[MLton] C99 runtime problems on Darwin

[Stephen Weeks]

> Including platform.h at the top of every C file (at least before any
> non-comments) and having it do the #defines first will definitely work.

OK, I've done this.  I left the following #defines in place, because I
wasn't sure if it made sense to move them to platform.h as well.

platform/cygwin.c:#define _GNU_SOURCE
platform/freebsd.c:#define _XOPEN_SOURCE 600
platform/linux.c:#define _GNU_SOURCE
platform/mingw.c:#define _GNU_SOURCE
platform/netbsd.c:#define _XOPEN_SOURCE 600
platform/openbsd.c:#define _XOPEN_SOURCE 600
platform/solaris.c:#define _XOPEN_SOURCE 600
gc.c:#define _GNU_SOURCE