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.
1 job for fix-FindfreeDiameter in 5 minutes and 2 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #855676784
CMakeCTest

00:05:02