[win32] make BuildSetup.bat et. al. more robust for jenkins builds

Jenkins seems to have problems with keeping a connection between the master and a slave open if there are longer periods without any console output.

These commits adjust BuildSetup.bat etc for win32 to provide more output by using MSBuild.exe (or devenv.com instead of devenv.exe) if possible and by running the ffmpeg configure, which doesn't produce any output on the console in the background and constantly writing dots to the screen until the ffmpeg build scripts are ready to provide some output of their own.

The former change is nice to have anyway IMO because it makes it possible to actually follow the build progress. The latter is a workaround (thanks to @Memphiz) which seems to work well enough but could do with something better (if possible). But as long as it fixes the problem at hand and gives us a working build slave again, IMO we don't have much choice.