Pass SimpleITK_* CMake variable from superbuild to external project

To remote the git hash from the nightly builds, the CMake variable
SimpleITK_BUILD_DISTRIBUTE needs to be defined in the actual SimpleITK
project. This is now accomplished introspectively examining and
passing all CMake variables which begin with SimpleITK_ from the
superbuild to the SimpleITK sub-project.

Change-Id: I1d83ab54797c05012155f74abb6bc7f025032e07
Tweak message for variables passed to ITK sub-project

Change-Id: I1e1c44dfe116a21e9794a2c166a6d667c819ab8c
Merge topic 'SIMPLEITK-349_RemoveGitHashFromNighlties' into next

a4a798b Tweak message for variables passed to ITK sub-project
8272ed5 Pass SimpleITK_* CMake variable from superbuild to external project