Fix for CLTs C++ issue

[Joshua Root]

For those who haven't been following the ticket, we've determined that 
the problem is caused by the CLTs installer not deleting everything left 
over from older versions when updating, combined with some unfortunate 
logic in how clang++ constructs its include paths. Fresh CLTs 
installations are therefore not affected, but ones that have been 
updated over time can be.

The proper fix will be hopefully be in future installers, but for now 
you can do it yourself by running:

sudo rm -rf /Library/Developer/CommandLineTools/usr/include/c++

- Josh