-fno-weak is unknown in the Clang 3.4 in current Xcode (5.1)

(cherry picked from commit dd06463eadd9fe5e3e33e01be3d1aa3bd7c33abc)
Signed-off-by: Tor Lillqvist <tml@collabora.com>

Change-Id: I10611e97c17de9c00b818e22ac88ffbb736aaf37
Reviewed-on: https://gerrit.libreoffice.org/8752
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>