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 <eric.engestrom@intel.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>

meson.build

@@ -25,8 +25,8 @@ project(
     [find_program('python', 'python2', 'python3'), 'bin/meson_get_version.py']
   ).stdout(),
   license : 'MIT',
-  meson_version : '>= 0.44.1',
-  default_options : ['buildtype=debugoptimized', 'c_std=c99', 'cpp_std=c++11']
+  meson_version : '>= 0.45',
+  default_options : ['buildtype=debugoptimized', 'b_ndebug=if-release', 'c_std=c99', 'cpp_std=c++11']
 )
 
 cc = meson.get_compiler('c')