Build script incorrectly assumes presence of `windres.exe` in MinGW environs.

```
    if(NOT CMAKE_RC_COMPILER)
        set(CMAKE_RC_COMPILER windres.exe)
    endif()
```

The above lines of `CMakeLists.txt` break compilation for android. The environment is MinGW, but we can't build a meaningful rc file. This should only be considered if the build environment is MingGW *and* the target environment is WIN32 *and* windres.exe is detected.