Use merge strategy -s ours to skip release commit on maint.

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.

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 8761e910.

* cmake: use CMake's internal variables for compiler info
* volk: void* can't be used for pointer arithmetic in MSVC, so cast

+ 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.

digital: replaces buffer copies with fir_filter_with_buffer to handle more sanely and with fewer extra copies.

volk: adds unaligned sse 32fc_32f_dot_prod_32fc, 16i_32fc_dot_prod_32fc, and 32fc_x2_dot_prod_16i proto-kernels.

If the given regex does not match any substring in a kernel name
then return 0 from run_volk_tests, otherwise execute the tests.
Defaults to a regex of .* to run everything, and no regex which
also runs everything for QA.

This reverts commit 9b37687d.