Fix warnings regarding use_texture_alpha

Add use_texture_alpha = "clip" for standard signs.
Break Minetest 5.4< compatibility.

depends.txt

@@ -1,7 +0,0 @@
-default
-intllib?
-screwdriver?
-streetspoles?
-streetlamps?
-cottages?
-prefab_redo?

description.txt

@@ -1 +0,0 @@
-Adds signs with readable text.

mod.conf

@@ -1,2 +1,6 @@
 name = signs_lib
+author = VanessaE
+depends = default
+optional_depends = intllib, screwdriver, streetspoles, streetlamps, cottages, prefab_redo
+description = Adds signs with readable text.
 min_minetest_version = 5.2.0

standard_signs.lua

@@ -14,7 +14,8 @@ signs_lib.register_sign("default:sign_wall_wood", {
 	allow_widefont = true,
 	allow_onpole = true,
 	allow_onpole_horizontal = true,
-	allow_yard = true
+	allow_yard = true,
+	use_texture_alpha = "clip",
 })
 
 signs_lib.register_sign("default:sign_wall_steel", {
@@ -35,7 +36,8 @@ signs_lib.register_sign("default:sign_wall_steel", {
 	allow_widefont = true,
 	allow_onpole = true,
 	allow_onpole_horizontal = true,
-	allow_yard = true
+	allow_yard = true,
+	use_texture_alpha = "clip",
 })
 
 minetest.register_alias("signs:sign_hanging",                   "default:sign_wood_hanging")