build: Do not eagerly disable checks on release builds

build: Do not eagerly disable checks on release builds

Release builds should only disable cast checks, to match what we used
to do back with the Autotools builds.

The Autotools build had an "--enable-debug=no" option, but that was
rarely used; Meson has debug, debugoptimized, release, and plain build
types, but we considered the "release" build types as the old "disable
all debugging code", which is not really accurate.

Disabling assertions and preconditon checks should be left to people
with constrained environments and/or packagers; they are supposed to
use the "plain" build type, and override the CFLAGS themselves.