This avoids violation of possible future CMake policy checks
regarding reserved/read-only variables.

Everything else depends on it, after all.

Which also happens to align the non-Debian specific variables
with the ones used by upstream CMake.

It's pretty much a simplified copy of the Linux script, lacking fancy colors.
Also, I had to drop Python testing, because it's not easy to pass the Python
module location to the script, and I have no pressing need to run the Python
tests at the moment.

The use of built-in CMAKE_CURRENT_LIST_DIR requires at least
CMake 2.8.3. This fix properly defines the variable to allow its
use in versions < 2.8.3 as well.

Fixes issue #4205.