Issue 22878: PEP 477 - "make install" and "make altinstall" integration

Issue 22878: PEP 477 - "make install" and "make altinstall" integration

The backport of ensurepip to 2.7.9 allows pip to optionally be installed
or upgraded using the bundled pip provided by the new ensurepip module.
The option can be specified persistently using the configure option:

    ./configure --with-ensurepip[=upgrade|install|no]

It can also be overridden on either the "install" or "altinstall" targets:

    make [alt]install ENSUREPIP=[upgrade|install|no]

For Python 2, the default option is "no" (do not install pip).