meson: Don't use get_option('buildtype')

We should directly check the values of the debug and optimization options instead.

get_option('buildtype') will return 'custom' for most combinations of -Doptimization and -Ddebug, but those two will always be set correctly if only -Dbuildtype is set. So we should look at those options directly.

For the two-way mapping between buildtype and optimization

• debug, see this table: https://mesonbuild.com/Builtin-options.html#build-type-options