modtool: fix finding gnuradio-runtime

pkg-config does not set variables when falls to default values, particulary include dirs,
thus we need check only if libs are set
modtool: fix detection of GNU Radio module directory
added tag_propagation_policy_t to block_gateway
digital: ofdm benchmark_tx/rx added lo_offset option and enabled external clock utilisation
digital: ofdm benchmark_tx/rx added --clock-source option for external clock utilisation
Merge remote-tracking branch 'mmueller/gateway__tag_propagation_policy_t'
Merge remote-tracking branch 'pinkavaj/gr_modtool_cmake_01'
Merge remote-tracking branch 'pinkavaj/gr_modtool_mod_detection_00'
digital: ofdm benchmark_rx fft conflict fix
Merge remote-tracking branch 'm0mik/digital_ofdm_benchmark_txrx_lo_offset_option_external_clock_utilisation'
uhd: if B2X0 is connected, change USRP config info strings to account for lack of daughterboard
blocks: agc3_cc bug fix for input values of zero
blocks: adding interleaved_char_to_complex block
Merge branch 'maint'
qtgui: fixes some bugs between qwt 5 and 6, some reorg.
ctrlport: only convert strings to bytes for probe2_b data.
grc: fixes module misnaming from 3.6 to 3.7.

Patch supplied by G33KatWork via github.
Merge branch 'maint'
blocks: interleaved_char_to_complex grc xml fix
digital: updated narrowband examples with --lo-offset and --clock-source options