meson: fix -Wno-foo argument testing

gcc and clang ignore `-Wno-foo` arguments nowadays, so we need to
test the positive variant instead.

meson.build

@@ -66,17 +66,24 @@ cc = meson.get_compiler('c')
 
 global_args = []
 global_args_maybe = [
-	'-Wno-unused-parameter',
-	'-Wno-shift-negative-value', # required due to Pixman
-	'-Wno-missing-field-initializers',
 	'-fvisibility=hidden',
 	'-DIN_WESTON',
 ]
+global_wnoargs_maybe = [
+	'unused-parameter',
+	'shift-negative-value', # required due to Pixman
+	'missing-field-initializers',
+]
 foreach a : global_args_maybe
 	if cc.has_argument(a)
 		global_args += a
 	endif
 endforeach
+foreach a : global_wnoargs_maybe
+	if cc.has_argument('-W' + a)
+		global_args += '-Wno-' + a
+	endif
+endforeach
 add_global_arguments(global_args, language: 'c')
 
 if cc.has_header_symbol('sys/sysmacros.h', 'major')