mesa: Have configure define NDEBUG, not mtypes.h.

mtypes.h had been defining NDEBUG (used by assert) if DEBUG was not
defined. Confusing and bizarre that you don't get NDEBUG if you don't
include mtypes.h.

... which is just what happened in commit bef38f62e.

Let's let configure define this for us if not using --enable-debug.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
diff --git a/configure.ac b/configure.ac
index beb7a7d..5fbb7bc 100644
--- a/configure.ac
+++ b/configure.ac
@@ -370,6 +370,8 @@
             CXXFLAGS="$CXXFLAGS -O0"
         fi
     fi
+else
+   DEFINES="$DEFINES -DNDEBUG"
 fi
 
 dnl