Parallel installation of SBCL and CCL versions

[kiselgra]

It would be nice if we could install both versions at the same time. Selection could either occur via names:

$ cm-ccl c ...

or via options

$ cm --ccl c ...

In both cases the actual generators (i.e. cm-c, etc) would have to be renamed or put in subdirectories (e.g. $prefix/bin/cm-ccl).

@lispbub Which one do you think would be better?

For both versions it should be possible to specify a default implementation and thus we would also have to be abel to query which was selected, e.g. for the use in scripts, by something like

$ cm impl
sbcl