[CMake] Passing a CMake list "as is" to a custom target

[Hauke Heibel]

Thank you for the feedback. I adopted using "::" as a separator and I
added en- and decode functions to my macro library. It's probably the
cleanest solution.

The only remaining comment is that opposed to the example David
posted, I get wrong results when escaping the semi-colon in the
regular expression. I did not look at the ExternalProject code but its
working for me and I am now fine with the solution.

Thanks again,
- Hauke