Noticed when trying to pass custom optimization flags, and hence setting
buildtype to plain, that warnings for different levels were not passed to
compiler.
This was a bit confusing since mesonconf still displayed "warning_level=3"
and -Werror was passed correctly due to "werror=true". So this change
aligns warning_level behavior with werror. That is, heed what is in
project() in meson.build but user can still override if necessary.
This is very useful to prevent compiler noise about invalid cflags; particularly
for the -mms-bitfields cflag which most libraries send to the mingw-gcc
compiler. More can be added later.
Minijackson