OS X Mavericks/Xcode5: Find libxslt and libxml2 headers

The development headers are no longer installed in a system location,
but only in the sysroot.

Change-Id: Ie9b85ff638c435e7e4d0a538f6e72d2cdbae88c8
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@digia.com>