contrib: Do not pass debug/optim flags to meson

This fixes the meson underscore prefix test, which misbehaves
when -g is passed, as it would detect the debug string without
underscore first and incorrectly report that no underscore
prefix for symbols is used.

Fixes build issues with dav1d, which relies on the underscore
prefix check.

See: https://github.com/mesonbuild/meson/issues/5482


Signed-off-by: Hugo Beauzée-Luyssen <hugo@beauzee.fr>