Make python install respect custom CMAKE_INSTALL_PREFIX

The default python install directory is the system path, even when CMAKE_INSTALL_PREFIX is set to a local user directory (or even to /usr/local).  I worked around the issue easily by setting PYTHON_CUSTOM_TARGET, but perhaps the following is a nicer way to handle this?  In general, the build system should never install anything into /usr IMO - that should be reserved for the package manager.  If the user really must, they can explicitly set CMAKE_INSTALL_PREFIX=/usr.

(Side note: It seems that doing this consistently in a cross-platform way may be fraught with trial and error, I suspect this stuff just breaks horribly on windows in either case.)