Support CMake 2.8.12.

[blowekamp]

This issue is in regards to the following build on the nightly ITK dashboard:
https://open.cdash.org/buildSummary.php?buildid=5053294

Is CMake 3.0+ really required? Can support for older ~2.8.12 be added?

[phcerdan]

Yes, I don't think we are using any 3.0+ feature. Changing it.

[phcerdan]

Pushed to gerrit: http://review.source.kitware.com/22598

[blowekamp]

Thank you!