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
blocks: adding a tags strobing block to help test tag stream.
blocks: modified tag_debug block to publish number of tags in it's input queue to ControlPort.

Also now respects display: off from GRC.
runtime: vmcircbuf_sysv_shm workaround for shmem race condition
Merge branch 'maint'
cmake: fixed some differences between handling Ice 3.4 and 3.5.
Merge branch 'maint'
build: fixes FindQwt cmake module to check that Qwt is >= 5.2.0.
Merge branch 'maint'
build: more FindICE fixes so it doesn't get confused with multiple installs of ICE.
blocks: moved type conversions to use Volk dispatchers.
modtool: fixes problem of using templates in OOT projects created with modtool.
Merge branch 'maint'
removed *out from sinks, *in from sources
examples: Some of the examples were updated to use 3.7 API, but the new module namespaces were never imported.
Merge branch 'maint'
Merge remote-tracking branch 'marcusmueller/improve_source_sink_c++_template'
docs: Fixes sphinx docs generating script to use any current LD_LIBRARY_PATH already set by the user.
Merge branch 'maint'