meson: Disable some warnings when asserts/checks are disabled

meson: Disable some warnings when asserts/checks are disabled

Some labels are only used when checks are enabled, and some variables
are only used when assertions and/or checks are enabled. Instead of
cluttering the code with extra #ifdefs, we silence those warnings:
they're harmless in this case. We already do this in Autotools.

Signed-off-by: Simon McVittie <smcv@collabora.com>