meson: fix warnings in build files

meson: fix warnings in build files

meson prefers that project-level options for Wall/Wextra/pedantic be
used, rather than hardcoding raw flags in add_project_arguments. If you
do the latter anyway, it raises a meson warning.

Set the default options for the project to use all this.

Also move the -Werror comment to the project default options with
appropriate format, but leave it commented out since it does not work.