bug#4218: Security assertion needs modification or more detail

[Chong Yidong]

> The docstring for compile-command says: "This variable is safe as a
> file local variable if its value satisfies the predicate `stringp'."
> I'd say this is arguable, as it can be set to an arbitrary command
> e.g. "send-me-all-your-passwords; make -k".

Thanks for the bug report.

I think the main risk occurs when the user has customized
compilation-read-command to nil, because then M-x compile does not issue
a prompt.  So, I've changed the predicate to consider compile-command
unsafe if compilation-read-command is nil.

We could be more aggressive and always consider compilation-command
unsafe, but I'm not sure that's warranted.  After all, there is the risk
that your makefile is malicious, anyway.