cmake: Added custom NSIS templates

* Unused "gnuradio Desktop icon" option removed
* GNU Radio Companion can now be accessed from the Start Menu
uhd: enhanced version of usrp_spectrum_sense.py
build: fixed GnuradioConfig files for OOTs to use.
docs: fixed warnings.
runtime: id docstring clearer
build: removed some old autotools-related documentation.
runtime: fix for thread safety issue during simultaneous use of stream tags and performance counters
Merge branch 'maint'
runtime: forgot to go back to the 'old' style using gruel.
Merge branch 'maint'
cmake: updated GnuradioConfig.cmake file to look for headers installed in 3.6.
modtool: added comment to modtool's generated root CMakeLists.txt file to describe use of GnuradioConfig.cmake.
gitignore: git now ignores the build/ directory
Merge remote-tracking branch 'bhilburn/gitignore_fix'
blocks: Added tag gate
digital: OFDM passes all tests
utils: fix modtool template output signature
Merge branch 'maint'
Merge remote-tracking branch 'martin/master-ofdm2'
doxygen: Update pfb introduction.