Windows: Make build.bash prefer MinGW-w32 over MinGW.

Windows: Make build.bash prefer MinGW-w32 over MinGW.

This is simply for licensing reasons. The 64-bit version
will be built with MinGW-w64 anyway (at least for now),
so using it also for 32-bit build allows using the same
copyright notice about the MinGW-w64/w32 runtime.

Note that using MinGW would require a copyright notice too,
because its runtime is not in the public domain either even
though MinGW's home page claims that it is public domain.
See <http://marc.info/?l=mingw-users&m=126489506214078>.