locally modifying `compilation-environment'

[Stephen Leake]

`compilation-start' uses `make-local-variable' to make
`compilation-environment' buffer-local in the created compilation
buffer.

That complains if it is let-bound, like this:

      (let ((compilation-environment ...)
            ...)

        (compilation-start cmd
                           'compilation-mode
                           (lambda (mode-name) (concat mode-name 
"-gnatinspect-overriding")))
        )

This gives a message:

    Making compilation-environment local to
    compilation-gnatinspect-overriding while let-bound!

I assume `compilation-environment' is made buffer-local so it will have
the same value for a rerun of the compile command in that buffer, which
makes sense.

But I don't see any other way to modify compilation-environment for this
compilation only.

compile-start-hook is run too late, so I can't set
compilation-environment there.

In this particular application, I can let-bind process-environment
instead (it is not made buffer-local); I don't care about re-running the
compile command. But that by-passes the purpose of
compilation-environment in general.

It seems compilation-start needs another parameter, for a local value of
compilation-environment. 

-- 
-- Stephe