gcc/g++ failures after xcode11 update

[Chris Jones]

>> (*) The package to add back /usr/include currently does not exist in 
>> 10.15 beta, so unless it reappears come final release this is going to 
>> be more of a problem there….
> 
> This package is not going to be coming back: 
> https://developer.apple.com/documentation/xcode_release_notes/xcode_10_release_notes#3035623
> 

Indeed, it is not at all unexpected to no longer have this option with 
Xcode 11.