VSOptions: Prepare for setting the INCLUDE env var for importC

[kinke]

Upstreaming from LDC, to pave the way for fixing Issue 24111.

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✗	24111	enhancement	[ImportC] fatal error C1034: stdio.h: no include path set

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#15780"

[kinke]

I don't have any old SDKs on my box, so couldn't verify the subdirs for those.

[kinke]

See https://github.com/ldc-developers/ldc/blob/8399e47ceb4dfbbcf1ca0b7cf934c04a864387b7/driver/tool.cpp#L297-L312 for how these 2 functions are to be used as base for setting up INCLUDE.