[packaging] Re-enable 'debug' checks in release builds

The original intent of using --enable-debug=no on release builds was to
improve stability by removing overly-aggressive asserts and checks that
could bring down important processes. This was misguided: I can't point
to one crash that happens because of an assert/check that wouldn't
otherwise crash, except when G_DEBUG is set in the environment.

Also, many applications implicitly depend on the functionality removed
by G_DISABLE_CHECKS to operate safely. Having that flag in our builds
turns warnings into crashes and could harm stability.

The only valid argument in favor of --enable-debug=no is performance,
but the benefit there is unclear and we don't use glib in
performance-sensitive paths. Other distributions seem to agree, and
don't set this option.

Note that the default is 'minimum', which includes casts, checks, and
asserts. --enable-debug=yes does much more, but it's generally only
enabled for unstable glib versions.