I'm trying to build the C++ port of zxing on Windows, but scons fails with:

```
cl : Command line error D8021 : invalid numeric argument '/Wextra'

```

I have both VS2010 and MinGW installed, and scons tries to use the MSVC compiler, even though the SConscript file assumes gcc and use gcc-specific parameters, which causes the error.

How can I tell scons to use MinGW instead?