fix defaults for mingw builds
* we have to set BOOST_USE_WINDOWS_H=1 because of this bug: http://stackoverflow.com/questions/18134148 * remove '/EHsc' and '/FORCE' when compiling with mingw instead of msvc
* we have to set BOOST_USE_WINDOWS_H=1 because of this bug: http://stackoverflow.com/questions/18134148 * remove '/EHsc' and '/FORCE' when compiling with mingw instead of msvc