Set default compiler to be the same that built dub

[ibuclaw]

Raising this PR as a bug - there are likely other things that need to change too, such as the help message.
" dmd (default), gdc, ldc, gdmd, ldmd"

It would be nice if the default compiler dub used was the same that built the application. So for me, I don't need to specify --compiler=gdc explicitly every time.

[s-ludwig]

Seems like a good intermediate solution, I'll fix the help message. For the full solution I'd then go with something like #298 (keeping the original compiler as the default).

[Geod24]

Actually, this has an issue: it defaults to the gdc, dmd, or dmd command, not the actual compiler that built it.
e.g I built dub using dub build --compiler=gdc-4.9, and when I invoked it, it called gdc (which is gdc-4.8 as I'm using Debian's package).
Which luckily was noticeable because gdc-4.8 SEGV on dub source code.

[s-ludwig]

@Geod24: I've modified the code and the build scripts now to use the actual binary name that was used to build DUB.

[JohanEngelen]

@s-ludwig It looks like this functionality has been lost. determineCompiler is not called anywhere relevant. Thanks for looking into this.

[s-ludwig]

I didn't remember the history for this anymore, but it was removed deliberately: #678

But I'd say that #895 could be a good possibility to achieve the same practical outcome - assuming that that's the goal. It would probably make sense to implement that in conjunction with #875, for which I couldn't really make my mind up so far (any opinions on that one are very much appreciated).

Another, possibly cleaner in terms of usual OS behavior, alternative would be to support a configuration file in the same directory as the dub executable, which could have a pre-filled "defaultCompiler" field.

[JohanEngelen]

Indeed, #895 is the goal. With #303 (this PR), it would still not solve that. Good pointer to #875 !

[jacob-carlborg]

I didn't remember the history for this anymore, but it was removed deliberately: #678

One reason I can think of why it was removed is to be able to use an optimizing compiler (ldc/gdc) when building binaries of Dub for a release but that shouldn't affect the default chosen compiler when running Dub.