compat: fixes for header handling with OpenBSD / NetBSD

Handle OpenBSD and NetBSD as FreeBSD / DragonFly are. OpenBSD would
need _XOPEN_SOURCE to be set to 700. Its simpler to just not set
_XOPEN_SOURCE.

    CC strbuf.o
strbuf.c:645:6: warning: call to undeclared function 'getdelim'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
        r = getdelim(&sb->buf, &sb->alloc, term, fp);
            ^
1 warning generated.

Signed-off-by: Brad Smith <brad@comstyle.com>
Reviewed-by: Collin Funk <collin.funk1@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: brad0

compat/posix.h

@@ -60,11 +60,11 @@
 # else
 # define _XOPEN_SOURCE 500
 # endif
-#elif !defined(__APPLE__) && !defined(__FreeBSD__) && !defined(__USLC__) && \
-      !defined(_M_UNIX) && !defined(__sgi) && !defined(__DragonFly__) && \
-      !defined(__TANDEM) && !defined(__QNX__) && !defined(__MirBSD__) && \
-      !defined(__CYGWIN__)
-#define _XOPEN_SOURCE 600 /* glibc2 and AIX 5.3L need 500, OpenBSD needs 600 for S_ISLNK() */
+#elif !defined(__APPLE__) && !defined(__FreeBSD__) && !defined(__NetBSD__) && \
+      !defined(__OpenBSD__) && !defined(__DragonFly__) && !defined(__MirBSD__) && \
+      !defined(__USLC__) && !defined(_M_UNIX) && !defined(__sgi) && \
+      !defined(__TANDEM) && !defined(__QNX__) && !defined(__CYGWIN__)
+#define _XOPEN_SOURCE 600 /* glibc2 and AIX 5.3L need 500 */
 #define _XOPEN_SOURCE_EXTENDED 1 /* AIX 5.3L needs this */
 #endif
 #define _ALL_SOURCE 1