Change default path of MinGW to C:\mingw32

The new 4.8.6 installer with MinGW-builds 4.8.2 asks for the directory it expects MinGW in, and puts "C:\MinGW" as default. However, the advertised MinGW-builds 7zip by default contains a directory named "mingw32".

I suggest to change the default MinGW path therefore to "C:\mingw32".