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The sum of two positive (resp. negative) FP numbers should always be
positive (resp. negative), even when the result is zero.

This commit also cleans a bit some other parts of the specification of FP
operations.

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

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

When sessions are saved and then loaded from disk, goal identifiers are
properly qualified. This is not the case when goals are still in memory
due to reloading a file in the ide. So merging can get confused since
several goals potentially share the same base name.

The date at which the configure script was run does not carry much
information, so get rid of it.