asdf: multiple compiler output directories

When compiling an application with different compilers or different
versions of the same compiler, it is sometimes desirable place
compiler output in different directories according to the particular
compiler.

For example if I had an application foo, with a single source
directory, I might have a directory structure like:

foo
foo/cmucl
foo/acl5
foo/acl6
...

Sometimes people have relied on different compilers using different
extensions for output files.  But this isn't always exactly
sufficient.  For example when going from acl5 to acl6, the fasl format
changed but the output extension used by the compiler is .fasl in both
cases.

How hard would it be to arrange for this kind of output segregation
using asdf?  I'm experiencing minor pain at the moment as I am working
with both acl5 and acl6.  Are people already doing this kind of thing?

-- 
John Michael Adams


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