Standardize on CMAKE_INSTALL_PREFIX

CMake uses `CMAKE_INSTALL_PREFIX` to let the user specify installation prefix.  This changes usage of `PROJECT_BUILD_RELEASE_DIR` to `CMAKE_INSTALL_PREFIX` while preserving backward compatibility.

To install to a custom path:

`cmake -DCMAKE_INSTALL_PREFIX=<some external location> ../../src`

If no `CMAKE_INSTALL_PREFIX` specified, installation prefix will be ../release as before.

cc: @scottpurdy This is part of a broader change affecting numenta/nupic re: numenta/nupic#1234