- Allow overriding from the environment as suggest in the comment, which
  didn't actually work. (eg.: OPENMCL

- Makefile: new test-scl target. Add SCL's fasl file extensions
  to the clean target.
- Remove SCL TODO item.
- New file: cffi-scl.lisp.
- New primitive type :long-double. Since it's only supported by
  SCL, it's not worth adding a no-long-double feature just yet.
- New tests for :long-double.
- Add information about SCL and the new :long-double type to
  the user manual.

- Use variables for the lisps' paths so that one can easily pick
  different versions to run the tests with. eg:
    CMUCL

Well, support for asdf setups that don't put fasl files next to
the source files. The Makefile will delete the .fasls directories
on make clean (that's where my setup puts the fasls) and bindings.lisp
will save the shared library's location at compile-time.

Add unit testing framework using RT.
Lots of manual and CFFI-SYS updates.
Add passing a double float to sprintf in examples.lisp.
Update the syntax of the nested structure example.
Add the INC-PTR function to the CFFI-SYS backends.
Add the shareable byte vector interface to the CFFI-SYS backends.
Started writing support for aggregate structure slots.  Not used yet.

Clean up CLISP support some, will need CVS version of CLISP to properly
handle looking up symbols in all loaded foreign libraries.

Add preliminary support for CMU CL (tested in 19b-pre on OS X).