new xcode build system and build failures

[Ryan Schmidt]

On Oct 1, 2018, at 09:00, Ken Cunningham wrote:

> To force Xcode10 to use the old build system (as it did before) add this flag to the xcodebuild args. Some ports need this in both the build and destroot args (aquaterm), others pass it in a different way (macvim). 
> 
> UseNewBuildSystem=NO
> 

It seems like we should add support for this flag to the xcode portgroup via a new option.