turn compiler warnings into errors

[kikofernandez]

Most of the compiler warnings are now turned into errors

[albertnetymk]

I think it's good idea to have this. In fact, that's the case in the first place. Uncommenting "FatalWarnings" should be enough.

It fails on my box. No idea why CI works.

[kikofernandez]

now builds! There were some warnings (now thrown as errors) that were not visible to me.

[albertnetymk]

Builds on my box. Since we are not interested in that three kinds of warnings, why not disabling them, using -Wno-...? -Wno-error=... would still report them on building.

[kikofernandez]

I think it is good to have them as warnings though. We were ignoring them in the first place and that's one of the reasons why I never saw that the static variable n_calls was never used (task library) and probably why @PhucVH888 or @EliasC never realised that the function scons_print is not used either.
I believe it's good to get the warning in place. We will catch more errors in the long run.

[albertnetymk]

OK, then we should not use -Wno-error=unused-... completely.

[kikofernandez]

Merging in 15 min