Mildly improve Ubuntu 20.04 compatibility

meson.build

@@ -30,11 +30,13 @@ dependencies = [
 	dependency('libjpeg', required : get_option('jpeg-qs')),
 	dependency('libwebp'),
 	dependency('libwebpdemux'),
-	dependency('libwebpdecoder'),
+	dependency('libwebpdecoder', required : false),
 	dependency('libwebpmux'),
 	# https://github.com/google/wuffs/issues/58
 	dependency('spng', version : '>=0.7.0',
-		default_options: 'default_library=static'),
+		default_options: 'default_library=static',
+		# fallback : ['spng', 'spng_dep'],
+	),
 
 	lcms2,
 	libraw,