Actually fix path of generated DMD

[CyberShadow]

As OS was never defined, it was substituted as an empty string in the G variable.

This is a fixup for #6873.

I don't see where OS is being set by the autotester (if anywhere) - if it's not, then it's still using the current path hierarchy instead of the intended POSIX one. Let's see what the autotester says.

[CyberShadow]

I don't see where OS is being set by the autotester (if anywhere)

OK, I see. The win32.mak makefile still creates a dmd\src\dmd.exe file, which the auto-tester still uses.

I've confirmed that this pull request fixes CyberShadow/Digger#58 (thus making the path actually the same as on POSIX), and the build seems to be succeeding on the auto-tester Windows hosts.

CC @RazvanN7

[PetarKirov]

Looks good.

[WalterBright]

In the future, please use more illuminating branch names than pull-20170618-002833 which gives no clue what this might be about. Something like path-of-DMD.

[CyberShadow]

Sorry, that's part of an automated part of my workflow, and is not going to change. Please look at the commits in the branch or the PR title or description.