Replace compiling python with host python.

We're going not to compile python for host. It's only needed
for shared build and setup/install phases.
Assume we already have host python which could be
invoked to do installation (no shared build, anyway).
Fixes zerovm/zpython2#1.

Disabling shared modules build and install broke `exec_prefix`
directory structure creating. It lead to broken python installation
during python runtime configuration step.

Python uses `lib-dynload` as landmark for detecting `exec_prefix`.
Now fixing it.
Replaced bunch of build scripts with one.

Get rid of separate configure/compile/rebuild scripts in favour
of one: 2build_zpython.sh.

Building zpython2 has now only 2 phases: building pgen and
compiling python itself.