Warn about the use of deprecated env() function

This patch enables warnings if one of the deprecate functions that rely
in the implicit use of the global Genode::env() accessor are called.

For the time being, some places within the base framework continue
to rely on the global function while omitting the warning by calling
'env_deprecated' instead of 'env'.

Issue #1987

repos/base/include/rm_session/connection.h

@@ -40,7 +40,7 @@ struct Genode::Rm_connection : Connection<Rm_session>, Rm_session_client
 	 * \deprecated  Use the constructor with 'Env &' as first
 	 *              argument instead
 	 */
-	Rm_connection()
+	Rm_connection() __attribute__((deprecated))
 	:
 		Connection<Rm_session>(session("ram_quota=%u", RAM_QUOTA)),
 		Rm_session_client(cap())