meson.build—Donʼt fail if warning_level=everything

We assume the meson configure variable warning_level can always be converted to int, but nowadays there is a standard 'everything' option, which if used was causing ninja builds to fail. While that level is not very useful due to all the dodgy C stuff we are built on top of, it should still work in case anyone wants to test particular files under it – so this fixes it, by changing 'everything' to the arbitrarily high 99.