meson: Restore implicit warning/debug/optimize flags for extensions

meson: Restore implicit warning/debug/optimize flags for extensions

Meson uses warning/debug/optimize flags such as "-Wall", "-g", and
"-O2" automatically based on "--warnlevel" and "--buildtype" options.
And we use "--warning_level=1" and "--buildtype=debugoptimized" by
default.

But we need these flags for Makefile.global (for extensions) and
pg_config, so we need to compute them manually based on the
higher-level options.

Without this change, extensions building using pgxs wouldn't get -Wall
or optimization options.

Author: Sutou Kouhei <kou@clear-code.com>
Discussion: https://www.postgresql.org/message-id/flat/20240122.141139.931086145628347157.kou%40clear-code.com