core: Adds a gr-log concept for easy logging at various levels.

Uses log4cxx and is optional. Macro based and macros are simple pass-throughs
if this dependency does not exist or if the gr-log is disabled.
docs: adding info on gr-log feature.
digital: Adds checks to enable use of gr-log in digital blocks.
Fixing up the gr-log capabilities.

Better handling of enable/disable function, easier integration with all components if log4cxx is or is not there. All components have been updated to be able to use logging. Docs update, too.
howto: adding gr_log checks to build.
log: Restructure config file handling for gr-log.

The xml config file is installed into prefix/etc/gnuradio.

A new LOG section is added to gnuradio-core.conf that has an item log_file to point to the log file to be used.

If the log_file is not found, we can now set the file to "" to use the BasicConfigurator for very simple console logging. This is what will be used during 'make test'.

In gr_fir_sysconfig_x86.cc, all std::cerr calls are replaced with gr-log to easily turn them on/off.
log: added a macro and routine to set log level from config file.

Also worked on documentation of logging features.
log: fixed gr_log to be usable in Python.

Works with log4cxx installed and not installed.
Merge branch 'master' into gr_log

Conflicts:
	CMakeLists.txt
	cmake/Modules/GrMiscUtils.cmake
	docs/doxygen/other/build_guide.dox
	gnuradio-core/CMakeLists.txt
	gr-digital/lib/CMakeLists.txt
blocks: converted peak_detector2 and regenerate to v3.7.
blocks: adding threshold, strech, and throttle to gr-blocks.
blocks: moved transcendental block to gr-blocks.
modtool: another fix in the SWIG generating code
Merge branch 'master' of git://github.com/gnuradio/gnuradio into grmodtool
Merge remote-tracking branch 'mbant/grmodtool'
blocks: fixed typo.
core: adding a mutex as a protected member of gr_block for use in protecting set and work function access to variables.

This is used in gr-analog's noise_source and gr-filter's fir_filter_XXX since these two cause particular problems (in next's channel model). This shows the use of the mutex for general protection when needed.
cmake: adding Boost version 104601 to list of bad Boosts.
Merge branch 'maint'
Update maintenance release version to 3.6.3.1