check for os and xcode version

[Helmut K. C. Tessarek]

I know I can check the OS release in portfiles with:

platform darwin X {

where X is the platform version.

It might be necessary to add fixes when OSX is Sierra/High Sierra AND
Xcode 9.

How can I do this in the portfile?

Cheers,
 K. C.

-- 
regards Helmut K. C. Tessarek              KeyID 0xF7832007C11F128D
Key fingerprint = 28A3 1666 4FE8 D72C CFD5 8B23 F783 2007 C11F 128D

/*
   Thou shalt not follow the NULL pointer for chaos and madness
   await thee at its end.
*/