build-sys: support --with-python[={2,3}]

 * we use pkg-config to get CGLAGS and LIBS, use package specific
   config (e.g. python-config) is non-sense.

 * default is to follow distribution and use pkg-config module name
   "python". This is probably symlink to python2.pc or python3.pc.

 * --with-python=2 forces to pkg-module "python2 >= 2"

 * --with-python=3 forces to pkg-module "python3 >= 3"

Signed-off-by: Karel Zak <kzak@redhat.com>
pylibmount: rewrite to be py2/py3 universal

Signed-off-by: Ondrej Oprala <ooprala@redhat.com>
Signed-off-by: Karel Zak <kzak@redhat.com>