Use premake5 beta2's externalincludedirs when available

As of the beta2 version of premake5, sysincludedirs has been deprecated
and replaced with/renamed to externalincludedirs, and continuing to use it
causes warnings to be emitted.

With this change, we now use externalincludedirs when available so as to
prevent the warnings, falling back to sysincludedirs when it's not to prevent
breakage with pre-beta2 versions of premake5.

Accepted By: sera
Fixes: #6785
Differential Revision: https://code.wildfiregames.com/D4980