As of the beta2 version of premake5, sysincludedirs has been deprecated in favour of externalincludedirs, and continuing to use it causes warnings to be emitted.
Whilst we don't (yet) use the beta2 version of premake5 ourselves, it is possible that those who use the --with-system-premake5 argument might do.
This revision changes things so that we use externalincludedirs when available to prevent the warnings, falling back to sysincludedirs when not to prevent breakage with the bundled version of premake5 and those using --with-system-premake5 with a system-installed version prior to beta2.
Note that updating the bundled version of premake5 to beta2 or newer is likely to be a requirement of D3127.