BUG: Create Python virtualenv at configure time. #95.

The virtualenv needs to available to install the tubetk packages for testing.
Merge pull request #96 from thewtex/Virtualenv-at-configure

BUG: Create Python virtualenv at configure time. #95.
BUG: Explicit link against PYTHON_LIBRARIES for Mac.
Merge pull request #98 from thewtex/MacPythonModuleLibrary

BUG: Explicit link against PYTHON_LIBRARIES for Mac.