grc: fix error message in console when using qtgui blocks
build: moving the location of the config.h file in the build tree so we can access it in sources as gnuradio/config.h.

Also uses the config.h to pull out if we've defined GR_PERFORMANCE_COUNTERS to handle the setup_pc_rpc function if perf counters are/are not compiled in.
cmake: Adds GR_CTRLPORT as a define in the root directory to populate config.h properly.
Merge branch 'maint'