build-sys: don't build python modules after --without-python

The modules should build just fine, but AM_PATH_PYTHON sets
pkgpyexecdir for us. Without that variable we don't know where to
install modules. In addition libtool tries an empty rpath, breaking
the build. Those issues could be fixed or worked around, but we
probably don't have many people who want to avoid using python binary,
but want to compile python modules. If such uses ever come up, this
issue should be revisited.