Fix GDC build on SemaphoreCI

[wilzbach]

Follow-up to #7714

For some reason SemaphoreCI didn't want to run on this PR and it turns out that gdmd doesn't support -lib.

See e.g. https://semaphoreci.com/dlang/dmd-2/branches/master/builds/391

[dlang-bot]

Thanks for your pull request, @wilzbach!

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Testing this PR locally

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[wilzbach]

FreeBSD failure looks unrelated -> https://issues.dlang.org/show_bug.cgi?id=18704

edit: yep, it was:

[ibuclaw]

it turns out that gdmd doesn't support -lib.

It does support -lib, so maybe you could be a bit more specific or have a look at the -vdmd output of the script and fix whatever is wrong. ;-)

[wilzbach]

This is the failing line for gdmd:

/home/seb/dlang/gdc-4.8.5/bin/gdmd -lib -of../generated/linux/release/64/backend.a abc.o

or have a look at the -vdmd output of the script and fix whatever is wrong

Sure - D-Programming-GDC/gdmd#5

Though we should still merge this PR here as the gdc release we use here (aka the "latest") is from 2016.

[ibuclaw]

The gdmd wrapper is only superficial, and currently the ci pulls the latest copy anyway.

[wilzbach]

The gdmd wrapper is only superficial, and currently the ci pulls the latest copy anyway.

It's locked to a specific commit:

https://github.com/dlang/installer/blob/2e3086a02305637d119e252e539e47f365228ec0/script/install.sh#L554

So an update is needed -> dlang/installer#316

While I could merge it at the installer, I don't have the permissions to deploy the installer script.
So I think we should still go with $(AR) for now ;-)