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.