boost port file does not properly include isysroot

[Joshua Root]

On 2012-1-31 09:05 , Ryan Schmidt wrote:
> 
> But since we do expose the option to change the SDK in macports.conf, we should strive to fix any problems that arise because of this.

I would just like to remind everyone that setting
macosx_deployment_target in macports.conf is undocumented and
unsupported. It's there because a few users find it useful and allowing
it wasn't too difficult or disruptive, since the MDT environment
variable and the sdkroot stuff already had to exist for other reasons.
It does not constitute a complete solution for cross-compiling ports.

- Josh