Add OS X 10.5 support to configure.ac

Change-Id: I16ea9850ae2dab0500940cec0a1873e6c5ccfdae
Reviewed-on: https://gerrit.libreoffice.org/7030
Reviewed-by: Douglas Mencken <dougmencken@gmail.com>
Tested-by: Norbert Thiebaud <nthiebaud@gmail.com>
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>