libxml2 w/o python --with-python=no on all systems

When building libxml2 without python enabled, pass "--with-python=no" to
configure on all systems, not only on FreeBSD. Omitting mentioned
configure option results in building libxml2 WITH python enabled. In
this case, configure is looking for python by itself in system paths.
With a rootless configuration, python includes from the host are accessed.
If those files (Python.h) are not available, building libxml2 fails.