fix camlp5 uninstall

[fdopen]

See #11440
I've noticed it, but didn't take it serious. 'rm -rf /' doesn't work with rm of coreutils or any other version of 'rm' I've seen. It will just print a warning. If this is not a universal feature, it's a very serious problem and this PR should be merged very soon.

[camelus]

✅ All lint checks passed 98b5ae5

• These packages passed lint tests: camlp5.7.03

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✅ Installability check (8323 → 8323)

[mseri]

I am not too happy with the fix. It may be useful as a fast workaround to get in, but this should be also fixed in the camlp5 makefile, setting the DESTDIR to some dummy tmp value or aborting the commands if DESTDIR and/or CAMLP5N are not set.

[thierry-martinez]

Camlp5 Makefile is now fixed (@roglo): camlp5/camlp5@d561693