use bundled (non-system) versions of libxml2 and libxslt for OS X <10.6

fixes fdo#78612 and fdo#77893

Change-Id: Iac36211095cc009a356f335a93a184ea5078214d
Reviewed-on: https://gerrit.libreoffice.org/9353
Reviewed-by: Tor Lillqvist <tml@collabora.com>
Tested-by: Tor Lillqvist <tml@collabora.com>