Rewrite the version test to check for "set_property(CACHE" availability

This command has been introduced in 2.8, see
  http://www.kitware.com/blog/home/post/82

But writing
  if (${CMAKE_VERSION} VERSION_GREATER 2.8)
is much more readable than
  if( NOT ${CMAKE_VERSION} VERSION_LESS 2.8 )
Let CMake GUI display OCE_BUILD_TYPE as a combo box

Apply the same trick as with OCE_MULTITHREAD_LIBRARY.
This applies only on generators working on a single configuration.
Merge pull request #396 from tpaviot/review/CMake-combobox-BUILD_TYPE

Let CMake GUI display OCE_BUILD_TYPE as a combo box