[gforth] Configuring multiple libccdirs

[Reepca Russelstein]

Currently there are two places gforth will check for already-existing
compiled wrapper code:

1. ~/.gforth/<machine-type>/libcc-named
2. $libccdir or, if that isn't set, the value of $libccdir at the time
gforth was compiled.

This presents an issue if we have wrapper code we want to go somewhere
else and want gforth to still be able to find its own wrapper code -
we can't set libccdir for this purpose. For example, if I want to
install a separate package that provides wrapper code - maybe I'll
call it gforth-glfw - I'd want to put it in
PREFIX/lib/gforth/site-forth/libcc-named.

Could we change libccdir to specify a search path instead of a single
directory, with build-time $libccdir implicitly prepended to that
path? I see that libcc.fs already uses a search path internally for
it. The only complication I could foresee is saving the build-time
path as opposed to a single string.

(if there's some configuration option that lets me do what I've
described and I've missed it, I'd love to hear about it!)