Remove $NTA reference

Supports https://github.com/numenta/nupic/issues/809.

This PR should be merged together with https://github.com/numenta/nupic/pull/917 (pip-installable). This is because `lib_cppregion.so` will be installed too. In the current implementation, this file is obtained from $NTA/lib.
Thanks for the clarification.

Perhaps this isn't the right place to discuss but shouldn't we be installing dynamic libraries to the standard OS library location? This is generally `/usr/local/lib` for non-system libraries but can also be `/usr/lib`, `/lib`, or `$HOME/lib` I believe.