Use consistent (with Ubuntu 14.04) build configuration

This PR has two changes:

* Adapts the configure opts and cflags to more closely match that of Ubuntu's system packages
* Sets ``PYTHON_CONFIGURE_OPTS`` and ``PYTHON_CFLAGS`` in ``/etc/profile.d/pyenv.sh``. This will allow people to have the same build options as travis used to built ``/opt/python`` if they invoke ``python-build`` themselves manually.