GNU Makefile for blackhole sources

Add an additional GNU Makefile for creating the bh-executable and blackhole.o1-module
from the sources. Linking to libgambcgsc.so should also be supported now
if libgambcgsc.a doesn't exist.

The blackhole.o1 module can be used to open blackhole-interpreter session without bh-executable.

e.g. gsc -i -e '(load "blackhole")' -