Alias `platform=x11` to `platform=linuxbsd` in SCons

This makes it possible for users to follow outdated documentation
and still get a working binary.

This closes #37367.

SConstruct

@@ -251,6 +251,14 @@ else:
         print("Automatically detected platform: " + selected_platform)
         env_base["platform"] = selected_platform
 
+if selected_platform in ["linux", "bsd", "x11"]:
+    if selected_platform == "x11":
+        # Deprecated alias kept for compatibility.
+        print('Platform "x11" has been renamed to "linuxbsd" in Godot 4.0. '
+              'Building for platform "linuxbsd".')
+    # Alias for convenience.
+    selected_platform = "linuxbsd"
+
 if selected_platform in platform_list:
     tmppath = "./platform/" + selected_platform
     sys.path.insert(0, tmppath)