common, libbladeRF: log.c support can be disabled at compile-time

CMake option added to disable log.c for libbladeRF. Adjustments made
to other files as needed to address warnings (becoming errors via
-Werror and /Wx).

Removed return values for log-related functions, as these were unused
and made it slightly more annoying to make this change. The libbladeRF
minor version has been bumped to note this change in the currently
unstable series v0.x.x series.