libbs2b 3.1.0 used.
Adapt to lowpass range extension introduced with libbs2b 3.1.0
Version bump to 3.1.0
Bring back @BS2B_CFLAGS@
Add missing GPLv2+ license file
Remove --with-effect-level leftover
Add THANKS and AUTHORS file
Much cleaner 95% solution, version bump up to 0.9.0