Increase warning level & Fix all warnings

Merged rahix requested to merge fix-all-warnings into master

As discussed in !357 (merged), we should increase the number of enabled compiler warnings to catch a lot more silent bugs. Increase the meson-global warning level to 2, but excluding the unused-parameter and old-style-declaration warnings. Additionally, disable warnings for libraries as we cannot fix that code anyway.

Fixes #192 (closed)

Cc: @fuchsi

Edited by rahix

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