Fix _XOPEN_SOURCE problem on DragonFly

As on FreeBSD, defining _XOPEN_SOURCE to 600 on DragonFly BSD 2.4-RELEASE
or later hides symbols from programs, which leads to implicit declaration
of functions, making the return value to be assumed an int.  On architectures
where sizeof(int) < sizeof(void *), this can cause unexpected behaviors or
crashes.
This change won't affect other OSes unless they define __DragonFly__ macro,
or older versions of DragonFly BSD as the current git code doesn't rely on
the features only available with _XOPEN_SOURCE set to 600 on DragonFly.

Signed-off-by: YONETANI Tomokazu <y0netan1@dragonflybsd.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-compat-util.h

@@ -55,7 +55,8 @@
 # else
 # define _XOPEN_SOURCE 500
 # endif
-#elif !defined(__APPLE__) && !defined(__FreeBSD__)  && !defined(__USLC__) && !defined(_M_UNIX) && !defined(sgi)
+#elif !defined(__APPLE__) && !defined(__FreeBSD__) && !defined(__USLC__) && \
+      !defined(_M_UNIX) && !defined(sgi) && !defined(__DragonFly__)
 #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