COMP: Change minimum supported MacOSX version from 10.4 to 10.5

The build system is now consistent with what is reported on
http://download.slicer.org

See #2025

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22382 3bd1e089-480b-0410-8dfb-8563597acbee
COMP: Explicitly require CMake >= 2.8.11.2 on Mountain Lion

See comment in the code for details.

Fixes #2025
Fixes #2022
See also #2957

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22383 3bd1e089-480b-0410-8dfb-8563597acbee