Using hashes instead of strings in dynamic tags. In case of collision, an
anomaly is raised. As there are very few tags defined in Coq code, this is
very unlikely to appear, and can be fixed by tweaking the name of the
dynamic argument.

This should be more efficient, as we did compare equal strings each time.
Using hashes in Summary, like the previous commit did with Dyn.
configure: improve last fix

 Let's avoid the "if a=$(cmd ...)" since:
 - unless being a shell expert, it's not obvious it's testing the exit code
   of cmd.
 - it's quite fragile, if you pipe cmd into a cmd2 you'll lose the
   exit code of cmd.

 Instead, we test the emptiness of the variable content afterwards
Remove some occurrences of obsolete operators
Fixing bug #3165.
configure: typo in my last commit
Small allocation improvement in Closure.