#11 config file: M4 include path for aclocal

[Joerg Faschingbauer]

on a per-profile basis (as always), provide a
dictionary key that lets one add directories to the m4
include path that is passed to aclocal.

even better yet, let one completely override the
built-in path. could do this by generally overwriting
that path, and by providing an interface to the path
that has been set up so far - about as in a shell rc
file, when the PATH variable is built. but that's
optional I think.