fix gr-audio osx:
+ use GNU Radio preferences file to set default input and output audio device, if provided;
+ use gr::logger for all non-debug messages;
+ case-insensitive string find with desired audio device name;
+ fixes buffer allocation bug with low sample rates;
+ allows using a specific (named) audio device, or the default;
+ handles the case when the selected audio device becomes unavailable (e.g., a USB stick is removed while in use);
+ if no audio device name is provided, uses the default audio device as found in System Preferences::Sound;
+ handles the case when the default audio device is in use, and the user changes that audio device in System Preferences::Sound, by internally resetting to use the newly selected audio device;
+ all non-Apple names are now lower_case, not CamelCase;
+ move osx_impl functions to gr::audio::osx, and use them correctly;
+ install osx_impl.h to expose gr::audio::osx functions, but iff OSX audio is enabled.
fix typo in getting default source (output) audio device name from gr::prefs
Merge remote-tracking branch 'michaelld/fix_gr_audio_osx' into master_osx
runtime: updates prefs to preserve format of preference file options that are in quotes.
qtgui: adds ability to set qss style sheet as a preference.
Merge branch 'master_osx'
Revert "docs: fixes issues with building docs when various components are not enabled."

This causes build problems on OS X. Reverting for 3.7.3 to solve the more general case. We can review this again at a later time.

This reverts commit 8761e9104239f03ba1346ad3ea47a446a81de01a.
cmake: changing where GR_CTRLPORT is defined to properly go into config.h.
cmake: Turning performance counters on at compile-time by default. Still default to off at runtime (modify gnuradio-runtime.conf file to change this).

Experiments showed that this combination adds no overhead to GNU Radio.