CMake: massage the FindfreeDiameter logic
- munge include paths only if they were found (although the munge doesn't apply if the string value is NOTFOUND) - don't *always* set version to UNKNOWN before trying to find it, because it obscures when freeDiameter isn't found at all: you get a "unsuitable version UNKNOWN" error rather than a "library not found" error This is a bit of spinning wheels for when freeDiameter comes with a proper (CMake) config instead.