Test --disable-buffer-useinline in GitLab CI

#2031 (closed) would have been caught early if we were testing the --enable-buffer-useinline compile-time switch in GitLab CI. We should probably make one build job use it - maybe the CentOS 8 one?

One obstacle is that such a build does not currently work with --enable-developer as the warnings issued are turned into errors. These warnings should thus be addressed first.

Edited Jul 28, 2020 by Michal Nowak

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