Fix CMake dependencies for libmosquittopp.

Enables parallel build mode.

Accepted without CLA after discussion with Eclipse Foundation legal
about very small patches that can only be implemented in one way.

See bug report for acceptance of Certificate of Origin.

Bug: https://bugs.eclipse.org/bugs/show_bug.cgi?id=463884


Also-by: Gianfranco Costamagna <costamagnagianfranco@yahoo.it>