cmake: distinguish '-Cpath' from '-C path' in source dir parsing

This results in the correct source directory being picked up in calls with

cmake sourcedir -C settings

and in a more appropriate error message when calling

mkdir build ; cd build ; cmake -C settings

(cmake will then complain about the current working directory not containing a CMakeLists.txt file rather than settings being a file and not a directory)

Also fix -D and -U in the same way.