[Issue 18864] Building 64-bit dmd on Windows results in a debug build. The release build doesn't work.

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18864

--- Comment #2 from Atila Neves <atila.neves at gmail.com> ---
Some of my issues seem to stem from the fact that the top-level win64.mak
defaults to making a debug version of dmd and doesn't forward all of the
relevant variables to the win64.mak in src/.

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