cmake: improve OOT projects find the installed GNU Radio.

1. Do not install FindGnuradio.cmake; everything is done through GnuradioConfig.cmake.

2. Fix a small bug with finding the PMT library if pkg-config is used.

3. Updated the documentation for how to use these tools in the OOT project.
cmake: adding library suffix variable for setting the cmake install directory.
Merge branch 'maint'