compat: fixes for header handling with OpenBSD / NetBSD

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>