Use DC env variable as the default compiler

[omerfirmak]

When DUB recursively invokes itself, it sets up the DC
env var with the compiler it is using. The callee dub should
use that compiler if it is not overriden with "--compiler".

DC=ldc2 dub build is equivalent to dub build --compiler=ldc2
DC=ldc2 dub build --compiler=dmd would use dmd

This addresses the issue 1 mentioned in #2012

[dlang-bot]

Thanks for your pull request and interest in making D better, @omerfirmak! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
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[omerfirmak]

Any ideas on how to unittest this?

[PetarKirov]

Probably there are cleaner approaches, but the first thing that comes to my mind is this:

void main()
{
    version (LDC)
        return 1;
    else version (DigitalMars)
        return 2;
    // and so on.
}

[Geod24]

Another way to test it is:

void main (string[] args)
{
    version (LDC)
        immutable expected = "ldc2";
    // ...
    assert(expected == args[1], format!"Expected '%s' but got '%s'"(expected, args[1]); 
}

[omerfirmak]

Thanks for the reviews! Added the test.

[omerfirmak]

Travis seems to be stuck.