See http://www.mingw.org .
Gloss mingwin as "Minimalist GNU for Windows" [L1 ]. Is "mingw32" an exact synonym? In any case, one often sees the latter, also. Some idiolects seem to favor "minimal" for "minimalist", also.
Most of the excitement linking mingwin and Tcl has to do with critcl and its potential, first to maintain portable extensions far more conveniently, and then to ... In this scheme, mingwin provides cross-generation and cross-linking (among other) capabilities.
DRH offers the application note, "Cross-Compiling Tcl/Tk 8.3.2 for WinNT Using Mingw" [L2 ].
SLB: The download page of the mingw site offers a bewildering (to me) array of packages. The two important ones are:
The gcc package also contains a compiler but without the assembler and doesn't seem to be needed.
As far as I know, configure does not support compilation with Visual C++.
When starting the shell (C:\msys\1.0\bin\sh.exe) give the switch --login, so $PATH is correctly setup.