Do not hardcode Wall

Currently there are warning flags hardcoded in the meson.build:

add_project_arguments('-Wall', '-Wundef',
    language: 'c')

This should not be done IMHO as meson has the warning_level builtin option to control compiler warning granularity and will probably get more fine-grained options for this in the future, possibly making our hardcoded options conflict with the ones selected by the user.

Instead what we can do is set a different default for warning_level.