On non-Windows systems, send logs to syslog & stderr.

OpenBTS and YateBTS fork a transceiver sub-process and redirect stderr and
stdout, which effectively hide any bladerf logs. For debugging time-critical
code, printf debugging (or logging) is simple yet practically essential.
Logging to syslog made opt-in

-DDENABLE_LIBBLADERF_SYSLOG to enable
init_fini.c added

A generic mechanism for initializing libraries without having to use any
specific pre-known function.