[stable] Deduplicate dflags and lflags

[Geod24]

Before:

After:

There are still some improvements needed (notice the duplicated -L-framework), but the diff / benefit ratio is already great.
Packed in another tiny cleanup to avoid endless CI retriggers.
Targeting stable because I'm working on propagating dflags downwards and obviously this will come in handy.

[dlang-bot]

Thanks for your pull request, @Geod24!

[Geod24]

I believe that was also a pet peeves of yours @UplinkCoder

[s-ludwig]

But -L-framework needs to be duplicated for every framework - won't this result in a wrong outcome?

lflags "-L-framework" "-LCoreServices" "-L-framework" "-LCoreFoundation"

should get -L-framework -LCoreServices -LCoreFoundation with these changes, meaning that "CoreFoundation" is passed to the linker as a normal file argument instead of as a framework name.

As far as I can see, this operates on the wrong level and needs to be reverted.

[Geod24]

Look at the second screenshot: It is still duplicated.
However, I didn't knew this bit about -L-framework, and I have no idea why this is still duplicated. I'll take a look.

[s-ludwig]

Yes, I've seen the screenshot, but that does not look like the outcome of the algorithm, but of some coincident external effect.

[s-ludwig]

By the way, I know how annoying it is to miss a release, but in this case it was quite lucky that it came too late for the latest release. It really shouldn't have gone to the stable branch ;-)