Motivation / Problem
If path to openttd source contains special characters, CMake may fail to generate the build files.
I solved an issue if path contains some regexp commands (example "Openttd(2++)").
Compilation fails for "Openttd(2++)" but not for "Openttd (2++)", seems to depend on how ADD_CUSTOM_COMMAND decides to quote commands, and we can't do much about that.
Checklist for review
Some things are not automated, and forgotten often. This list is a reminder for the reviewers.