Moved AMP_QUERY_VAR definition to amp_init and filtered it.

Checking if the constant is defined might not be enought for some environments, where the plugins are loaded much earlier than the theme.
Using a filter when defining the constant allows to change the value on the theme.

amp.php

@@ -11,11 +11,6 @@
  * License: GPLv2 or later
  */
 
-// Let users define their own feed slug
-if ( ! defined( 'AMP_QUERY_VAR' ) ) {
-	define( 'AMP_QUERY_VAR', 'amp' );
-}
-
 define( 'AMP__FILE__', __FILE__ );
 define( 'AMP__DIR__', dirname( __FILE__ ) );
 
@@ -38,6 +33,8 @@ function amp_init() {
 		return;
 	}
 
+	define( 'AMP_QUERY_VAR', apply_filters('amp_query_var', 'amp' ) );
+
 	do_action( 'amp_init' );
 
 	load_plugin_textdomain( 'amp', false, plugin_basename( AMP__DIR__ ) . '/languages' );