Fix DMD path to ../generated

[wilzbach]

So I'm trying to move ahead with the ddmd -> dmd transition (dlang/dmd#7135) and due to braddr/at-client#4 we are stuck at the moment.

Hence, this PR tries to set the DMD binary path by default to one in the generated directory. As I have no Windows machine, I'm doing this more or less blindly, but it looks trivial enough.

See also: dlang/phobos#5892

[dlang-bot]

Thanks for your pull request, @wilzbach!

Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

[PetarKirov]

The Win_32_64 build failed with:

Error on line 6: expecting target : dependencies

which doesn't make sense to me. On the other hand, the phobos build went without a hitch, so the problem may go away.

[rainers]

Error on line 6: expecting target : dependencies

That's usually the error message that appears when patching the makefiles by the autotester fails. @braddr, please remove that.