Clang should use -isystem and -iquote instead of -I

[markfinal]

With Xcode 10, any angle bracket includes fail if using -I. Must use -isystem.
So might as well switch over.

[markfinal]

Reopening, since it feels a little too soon (pre-proper SDK creation) to be doing this.

[markfinal]

Some GCC warnings have appeared since reverting the change, so using angle brackets suppresses warnings.
I wonder if there needs to be an option to revert all use of include search paths to -I.