Fix and enable all the warnings

With 1.7.0 we have a few new deprecation warnings pop-up. I left them since they're not "too noisy" yet after rebuilding/packaging waffle a few times I realised how annoying they are. Plus the bad precedent of having any warnings in the log.

So for 1.7.1 let's try to:

• silence the deprecation warnings
• fix all other warnings - check release/debug meson/cmake builds
• enable -Wall -Wextra -Wpointer-arith
• optionally enable -Wmissing-prototypes -Wmissing-declarations -Wvla -Winit-self -Wundef
• enable -Werror in the CI