The GDBM wrapper.

This is a drop-in replacement for the gdbm microcode module and
runtime/gdbm.scm.  It is not part of the core build and can be built
outside the core build tree.  There is no ./configure script yet.  If
you know you have libgdbm installed, you should win with this command:

    make all check install

The install target copies a shared library shim and compiled Scheme
files into the system library path, and re-writes the optiondb.scm
found there.  You can override the default command name "mit-scheme"
(and thus the system library path) by setting MIT_SCHEME_EXE.

The option is currently called GDBM2 until GDBM no longer refers to
the original microcode module.  You will need to import the bindings
you want to use.  They are not exported to the global environment
because they would conflict with the exports from (runtime gdbm).
