[PATCH] 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 seems to be caused by a change to the system headers which
results in the _XOPEN_SOURCE macro now having prioity over the
_GNU_SOURCE and _BSD_SOURCE macros (they are simply ignored).
This in turn leads to the declarations of the above functions
to be suppressed.

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>
---
 git-compat-util.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)