Fix _XOPEN_SOURCE problem on DragonFly

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>