[CMake] QT_IMPORTS_DIR when dit does not exist yet

[Jonathan Riddell]

Some Ubuntu developers are having an issue where QT_IMPORTS_DIR is not defined when that directory does not yet exist.

https://bugs.launchpad.net/ubuntu/+source/cmake/+bug/894805

I have worked around it with http://paste.kde.org/188366/ but I
suspect I'm not understanding what that code does and it's not a
proper fix.  Any thoughts on a better way?

Jonathan