Enable Doxygen to generate documation for .cmake files

Using a perl script as a filter for .cmake files, Doxygen can now generate documentation for the cmake files.

Currently only functions and macros are included in the documentation.

Note that the documentation block in the current cmake files starts with #[==[.md. However, Doxygen has built in support for markdown format, thus not needing the .md marking. We can either modify the documentation or let the filter discard the markings.

With this filter, it is possible to write standard Doxygen command and labels like @param or \brief. They will just act like regular Doxygen comment blocks.