build: add -Wundef

There is no case in pre-processor directives where would like to have
undefined identifiers be silently replaced with a zero. This warning can
discover typos and forgotten includes.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

meson.build

@@ -63,6 +63,7 @@ global_args_maybe = [
 	'-Wno-shift-negative-value', # required due to Pixman
 	'-Wno-missing-field-initializers',
 	'-Wno-pedantic',
+	'-Wundef',
 	'-fvisibility=hidden',
 ]
 foreach a : global_args_maybe