mingw_cross_toolchain.cmake: set the MINGW variable to True

Setting MINGW to True is important to let cmake set some variables such
as CMAKE_FIND_LIBRARY_PREFIXES and CMAKE_FIND_LIBRARY_SUFFIXES from
Platform/Windows-GNU.cmake

I wasted a lot of time on this with find_library() not finding libav
libraries compiled for MingW whith the ".dll.a" suffix.