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>