Makefile: do not set setgid bit on directories on GNU/kFreeBSD

Makefile: do not set setgid bit on directories on GNU/kFreeBSD

The g+s bit on directories to make group ownership inherited is a
SysVism --- BSD and most of its descendants do not need it since they
do the sane thing by default without g+s.  In fact, on some
filesystems (but not all --- tmpfs works this way but UFS does not),
the kernel of FreeBSD does not even allow non-root users to set setgid
bit on directories and produces errors when one tries:

	$ git init --shared dir
	fatal: Could not make /tmp/dir/.git/refs writable by group

Since the setgid bit would only mean "do what you were going to do
already", it's better to avoid setting it.  Accordingly, ever since
v1.5.5-rc0~59^2 (Do not use GUID on dir in git init --share=all on
FreeBSD, 2008-03-05), git on true FreeBSD has done exactly that.  Set
DIR_HAS_BSD_GROUP_SEMANTICS in the makefile for GNU/kFreeBSD, too, so
machines that use glibc with the kernel of FreeBSD get the same fix.

This fixes t0001-init.sh and t1301-shared-repo.sh on GNU/kFreeBSD
when running tests with --root pointing to a directory that uses
tmpfs.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>