[CMake] Display all possible options in a CMakeLists file, like what we do with configure --help

[Tschijnmo Tschau]

Hi!

Sometimes different code has very different sets of options to tweak
in the building process. So when building a package with GNU
autotools, usually the first thing I do is ``configure --help'' to
list all the possible options that I can change. I am a little curious
if there is something quite similar to this kind of bahaviour in cmake
because it is really handy. Thank you so much for your help! Some
CMakeLists files from some codes do not seem to have been documented
about their options, so a convenient way to find them out seems to be
helpful.

Regards

Tschijnmo