warnings with gcc9.0.1 and clang 8

[Dmitri A. Sergatskov]

I have upgraded fedora's buildbot to fedora 30  that comes with

gcc version 9.0.1 20190312

and

clang version 8.0.0

Those give quite a few new warnings.

See:

http://buildbot.octave.org:8010/#/builders/9/builds/853/steps/5/logs/stdio

and

http://buildbot.octave.org:8010/#/builders/11/builds/854/steps/5/logs/stdio

(and may be newer)

I am not sure how serious are those, perhaps somebody more qualified can have a look.

Dmitri.

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