The dev-lang/tcl ebuild tries to build TCL in the /unix directory regardless of what system it is building for. In most cases it is safe to assume that you're building for some form of UNIX, but when using mingw to do the building, you're building for Windows, and should be building in the /win directory. Now, I'm not up on my ebuild-fu, but this:
Needs to change to something a bit more dynamic. I'd have tried my hand at playing with it, but I ran into a "Digest verification failed" issue that prevents me from manipulating this ebuild:
Calculating dependencies \ * Digest verification failed:
* Reason: Filesize does not match recorded size
* Got: 4005
* Expected: 3964 ... done!
>>> Verifying ebuild manifests
!!! Digest verification failed:
!!! Reason: Filesize does not match recorded size
!!! Got: 4005
!!! Expected: 3964
I wouldn't mind learning how to sandbox an ebuild like this and tweak it so that it works, since I'll probably be building a lot of stuff for Windows in the near future... but I'm not seeing any information on how to do that yet. I do see how to build your own ebuild, but not how to modify&test one that's already in portage.
Thank you for any suggestions, or perhaps... a fix?
Thank you for the report. Regarding "learning how to sandbox an ebuild"
I have had very good experience with the gentoo IRC channels  with questions like this. As you know, we can not discuss this in a bugticket.
Regarding the digest, you need to update the manifest .