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>

git-compat-util.h

@@ -75,7 +75,8 @@
 # endif
 #elif !defined(__APPLE__) && !defined(__FreeBSD__) && !defined(__USLC__) && \
       !defined(_M_UNIX) && !defined(__sgi) && !defined(__DragonFly__) && \
-      !defined(__TANDEM) && !defined(__QNX__) && !defined(__MirBSD__)
+      !defined(__TANDEM) && !defined(__QNX__) && !defined(__MirBSD__) && \
+      !defined(__CYGWIN__)
 #define _XOPEN_SOURCE 600 /* glibc2 and AIX 5.3L need 500, OpenBSD needs 600 for S_ISLNK() */
 #define _XOPEN_SOURCE_EXTENDED 1 /* AIX 5.3L needs this */
 #endif