Respect CMAKE_INSTALL_PREFIX in python lib path (#1325)

* Respect CMAKE_INSTALL_PREFIX in python lib path

* Set `base` and `platbase` to the CMAKE_INSTALL_PREFIX, if defined,
when asking Python for the default python lib install path.

* Differentiate between explicit and default prefix.

* Test for defaulted CMAKE_INSTALL_PREFIX, in which case, use the system
python site packages path, so that it's overall less excitingly awful in
python virtual environments. If you want /usr/local, set it.