Fix an "implicit function definition" warning.

The function at issue being initgroups() from the <grp.h> header
file. On Cygwin, setting _XOPEN_SOURCE suppresses the definition
of initgroups(), which causes the warning while compiling daemon.c.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git-compat-util.h

@@ -47,9 +47,15 @@
 #include <netdb.h>
 #include <pwd.h>
 #include <inttypes.h>
+#if defined(__CYGWIN__)
+#undef _XOPEN_SOURCE
+#include <grp.h>
+#define _XOPEN_SOURCE 600
+#else
 #undef _ALL_SOURCE /* AIX 5.3L defines a struct list with _ALL_SOURCE. */
 #include <grp.h>
 #define _ALL_SOURCE 1
+#endif
 
 #ifndef NO_ICONV
 #include <iconv.h>