Export  SimpleITK_BUILD_DISTRIBUTE and python threads

Add as advanced cmake variables  SimpleITK_BUILD_DISTRIBUTE and
SimpleITK_PYTHON_THREADS.

Change-Id: I0f805f6390f1a0f4c0aab4cbb30fc25992b16c6b
Merge topic 'SIMPLEITK-436_ExposeHiddenCMakeVariables' into next

b432421 Export  SimpleITK_BUILD_DISTRIBUTE and python threads