Increase warning level & Fix all warnings
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