meson: only turn on Mesa's DEBUG for buildtype==debug

As discussed in this thread:
https://lists.freedesktop.org/archives/mesa-dev/2017-November/175104.html

Cc: Dylan Baker <dylan@pnwbakers.com>
Signed-off-by: Eric Engestrom <eric.engestrom@imgtec.com>
Acked-by: Andres Rodriguez <andresx7@gmail.com>
Reviewed-by: Chad Versace <chadversary@chromium.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Tested-by: Chad Versace <chadversary@chromium.org>

meson.build

@@ -349,8 +349,8 @@ if cc.get_id() == 'gcc' and cc.version().version_compare('< 4.4.6')
   error('When using GCC, version 4.4.6 or later is required.')
 endif
 
-# Define DEBUG for debug and debugoptimized builds
-if get_option('buildtype').startswith('debug')
+# Define DEBUG for debug builds only (debugoptimized is not included on this one)
+if get_option('buildtype') == 'debug'
   pre_args += '-DDEBUG'
 endif