[Issue 19485] Add more predefined versions

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19485

--- Comment #4 from Iain Buclaw <ibuclaw at gdcproject.org> ---
(In reply to Iain Buclaw from comment #3)
> Possibly a job for dub, not the compiler.
That is to say, when it comes to discovering variants of the main
platform-specific versions.

Otherwise I only see the list of CPUs growing, rather than platforms - Linux,
HPUX, Solaris, Windows, OSX, AIX, OpenBSD, NetBSD, FreeBSD, VMS... that's
basically your lot, right? Others are either too obscure, or fork their own
toolchain instead of integrate upstream (such as Haiku).

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