Provide a simple API for loaded objects to interact with GNU make. I still
won't guarantee that this API won't change but it's much closer to something
that's supported and provides easy-to-use interfaces with a public header
file.
This allows us to create new functions without changing function.c.
You still have to modify the GNU make code (for now) though: this is
simply a preliminary step to possibly allowing make to load modules.
Modify the Guile integration to use this method rather than ifdefs
in function.c.
On configure-enabled systems, configure will detect Guile installed
(using pkg-config, which is how GNU Guile is distributed) and enable
it if so.
On all non-configure-enabled systems, currently, the default is for
Guile support to be disabled.
