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
channels: Added the hardware impairments modeling tools from Matt Ettus' GRCon11 talk.

Python blocks part of the gr.channel module.

Examples are in gr-channels/examples (demo_gmsk, demo_qam, demo_ofdm, and others).
qtgui: Added histogram sink.
channels: using new qtgui histogram sink in quantization demo.
docs: added more documentation to the channel models page explaining the diffenerent models available.

Includes discussion of the new HW impairments model that is Pytho-only.
qtgui: improved handling of scale in histogram plots.

Mouse scroll zooms in/out horizonatally. Added auto-scale feature, can set x-min and max in constructor.

Mouse scroll in constellation plot now does both axes at the same time.
channels: updated quantization demo for new histogram constructor.
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'