git-compat-util.h: don't define _XOPEN_SOURCE on cygwin

git-compat-util.h: don't define _XOPEN_SOURCE on cygwin

A recent update to the gcc compiler (v4.8.3-5 x86_64) on 64-bit
cygwin leads to several new warnings about the implicit declaration
of the memmem(), strlcpy() and strcasestr() functions. For example:

  CC archive.o
  archive.c: In function 'format_subst':
  archive.c:44:3: warning: implicit declaration of function 'memmem' \
    [-Wimplicit-function-declaration]
     b = memmem(src, len, "$Format:", 8);
       ^
  archive.c:44:5: warning: assignment makes pointer from integer \
    without a cast [enabled by default]
     b = memmem(src, len, "$Format:", 8);
       ^

This is because <string.h> on Cygwin used to always declare the
above functions, but a recent version of it no longer make them
visible when _XOPEN_SOURCE is set (even if _GNU_SOURCE and
_BSD_SOURCE is set).

In order to suppress the warnings, don't define the _XOPEN_SOURCE
macro on cygwin.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>