modtool: removing RTLD_GLOBAL hack for swig in gr-modtool.

This was removed from GNU Radio a while ago due to newer issues. Looks like it's affecting OOT modules as well.
volk: add check for posix_memalign for OSs that do not directly set _XOPEN_SOURCE or _POSIX_C_SOURCE; define and use the new variable when found.
logger: adding new configure_default_logger function to make it easier to get log file info on logger setup.
cmake: adding a function to allow components to reference submodules for display.

Mostly designed for gr-audio to print out which audio machines are enabled in the build.
audio: modified the audio registration system.

Instead of static functions and macros, this just uses simple ifdefs in the code to register new audio components if cmake found them. The code is more complicated for the developer, if we ever add a new audio machine. But this allows us to use gr-audio in statically linked cases.
volk: simplifies and fixes thread safety issue in volk_malloc/free.
Merge branch 'maint'
Merge remote-tracking branch 'arya/audio_registration'