Environment: set LANGUAGE variable both with LC_MESSAGES

As LANGUAGE takes precedence on LC_MESSAGES variable sometimes

If LANG is not set to C now en locale really should be used for messages
by gcc. This fixes parsing of gcc header paths and error messages on non-
english locales

Change-Id: I96db16b555b8ff22b041b6e965558b342ef1a378
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>