Move to c++17

[mgautierfr]

Fix #757

All our compilers should handle c++17. Let's move on.

This PR also (re)add werror=true and fix different warnings (raised differently by different compilers)

[codecov]

Codecov Report

Patch coverage: 60.00% and project coverage change: +0.05% 🎉

Comparison is base (66b3b7f) 57.47% compared to head (51947a8) 57.53%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #819      +/-   ##
==========================================
+ Coverage   57.47%   57.53%   +0.05%     
==========================================
  Files          98       98              
  Lines        4541     4540       -1     
  Branches     1910     1907       -3     
==========================================
+ Hits         2610     2612       +2     
  Misses        674      674              
+ Partials     1257     1254       -3     

Files Changed	Coverage Δ
include/zim/archive.h	94.82% <ø> (ø)
src/cluster.cpp	35.29% <ø> (+0.51%)	⬆️
src/fileimpl.cpp	47.95% <ø> (ø)
src/tools.cpp	53.65% <0.00%> (ø)
src/writer/contentProvider.cpp	58.33% <0.00%> (ø)
include/zim/search_iterator.h	100.00% <100.00%> (ø)
include/zim/suggestion_iterator.h	87.50% <100.00%> (ø)
src/writer/dirent.cpp	60.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[veloman-yunkan]

Pre-approving. Fixup commits must be squashed. Squashup commits (if any) must be fixed.