scons: Reorder options for clarity

Also prefix all thirdparty-related toggles with `builtin`.

(cherry picked from commit cc95d4448c)

modules/mpc/SCsub

@@ -6,7 +6,7 @@ Import('env_modules')
 env_mpc = env_modules.Clone()
 
 # Thirdparty source files
-if (env["libmpcdec"] != "system"):  # builtin
+if (env['builtin_libmpcdec'] != 'no'):
     thirdparty_dir = "#thirdparty/libmpcdec/"
     thirdparty_sources = [
         "huffman.c",