Added XBPS_ARCH environment var to override uname(2) machine result.

include/xbps.h.in

@@ -1,5 +1,5 @@
 /*-
- * Copyright (c) 2008-2013 Juan Romero Pardines.
+ * Copyright (c) 2008-2014 Juan Romero Pardines.
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -561,7 +561,8 @@ struct xbps_handle {
 	/**
 	 * @var native_arch
 	 *
-	 * Machine architecture, as returned by uname(2)::machine.
+	 * Machine architecture, defaults to uname(2)::machine
+	 * if XBPS_ARCH is not set from environment.
 	 */
 	char native_arch[16];
 	/**