So either a conversion from Unix LF to Windows CR+LF on those files is needed, or just use mingw to compile it using "$ make -f Makefile.mingw init upnpc-static" from msys instead of Windows command prompt.
Miniupnpc compiles fine from both msys and cmd producing identical binaries, so it's just a matter of preference which one you compile from.
Well, that is correct for now, but since the package contains all files with the Unix style end-of-line, I prefer to use MSYS on Windows, so I am sure that nothing breaks now, nor will in the future.
Also note that one of the batch files in the miniupnpc folder is named 'mingw32make.bat', very similar to the command that you instruct to use, 'mingw32-make', if the user has Windows CLI completion enable, and type the first few letter then auto-complete, that batch file will be used causing problems... also in Windows batch file can be called without the .bat extension, making this one even more similar to the actual command that we want to use.
... but yes for now is just matter of preference.