Add more compilation flags

[slaperche-scality]

In order to keep a good level of code quality, this PR introduces makes Clang and GCC stricter.

The set of flags are the ones that were used on the DCSS project.

The next step will be to enable more lint check in clang-tidy, but that PR is already big enough as it is.

Also, before moving onto the linter, there are still two more warnings that I want to enable: -Wconversion and -Wshadow. Those will require a litlte bit more rework so they will be done in a dedicated PR:

• -Wconversion: it's probably wise to tackle https://github.com/scality/quadiron/issues/243 first
• -Wshadow will require a bit of wide renaming (but in the end will improve the code readability) and I'll probably rework the visibility of some fields on the way (too many public fields in some of our classes right now).

[slaperche-scality]

@lamphamsy It would be great if you could review the changes in the commit "fix Doxygen documentation (flagged by -Wdocumentation)" carefully.

Most of the errors flagged by -Wdocumentation were about missing description for function parameters. I fixed those by either:

• adding a description when I clearly understood what's the role of the parameter
• adding a not-so-useful-but-not-wrong description when it wasn't clear to me
• removing the documentation of the parameters

If you have suggestions for those, I would be more than happy to update this commit with better descriptions/documentation :slightly_smiling_face: