Many of the structure offsets and symbolic constants necessary to do
FFI vary between architectures and operating systems.  To avoid a
maintenance nightmare, we derive them automatically by creating and
running a small C program.  The C program is created by
def-to-lisp.lisp with input from a GROVEL-CONSTANTS-FILE

The ASDF component type GROVEL-CONSTANTS-FILE has its PERFORM
operation defined to write out a C source file, compile it, and run
it.  The output from this program is Lisp, which is then itself
compiled.

How to use it from your own system

1) Create a Lisp package for the foreign constants/functions to go into.
It needs to use SB-GROVEL and SB-ALIEN

2) Make your system depend on the "sb-grovel" system

3) Create a grovel-constants data file - see example-constants.lisp in
this directory

4) Add it as a component in your system. e.g.

(defsystem sbcl-hemlock
    :depends-on (sb-grovel)
    :components
    ((:module "sbcl"
	      :components
	      ((:file "defpackage")
	       (sb-grovel:grovel-constants-file "example-constants"
						:package :sbcl-hemlock
						)))))

Make sure to specify the package you chose in step 1

5) Build stuff

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Note that we assume that the C type char has 8 bits.