driver: refine handling of --target.
    
    * for a --target argument that precedes the compiler argument, as in:
      slibtool --target=xxx --mode=compile cc
    
    --> set an explicit (cross-)target,
    --> but do not pass the argument to the compiler.
    
    * for a --target argument that follows the compiler argument, as in:
      slibtool --mode=compile cc --target=xxx
    
    --> set an explicit (cross-)target,
    --> and also pass the argument to the compiler.