ffi/build.scm.in: Support building shims separately.

ffi/build.scm.in: Support building shims separately.

A handful of new top-level bindings make a portickle Makefile.

gdbm-shim.so: gdbm-shim.o gdbm-adapter.o
	echo "(link-shim)" | mit-scheme --batch-mode -- -o $@ $^ -lgdbm

The new procedures (generate-shim, compile-shim, link-shim and
install-shim) autoload ffi/build.scm, which captured the build
configuration of the machine.  Compile-bundle and install-bundle are
also provided.