meson: disable asserts by default on release builds

By the time Mesa 18.3 comes out (probably December '18), Meson 0.45 will
be 9 months old (March '18), so I think this is reasonable.

(btw, the currently-required Meson 0.44.1 was released less than 12 days
 before 0.45, so we're really not bumping by much.)

Currently, the Meson versions in the major distributions are:
Arch:     ships 0.47.2
CentOS:   7 ships 0.47.1
Debian:   stable ships 0.37.1, so it hasn't been usable in a long time.
          everything more recent ships 0.47.2
Fedora:   28 ships 0.45.1
FreeBSD:  ships 0.46.1 (ports)
Gentoo:   ships 0.46.1
OpenSUSE: 15 ships 0.46
Ubuntu:   18.04 ships 0.45.1

Signed-off-by: Eric Engestrom <[email protected]>
Reviewed-by: Dylan Baker <[email protected]>

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