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Moreover, this commit also performs the emacs compilation at build time
rather than install time. It also uninstalls why3.elc systematically.

Since this might break some cases relying on opaqueness of
type variables occuring only in return types, opaqueness
annotations are now allowed there as well.

The goal is that most recent rules will shadow older ones.
Previous behavior was the converse.

Use an optional 'words argument to regexp-opt instead of explicitly
surrounding the result with special chars.