Check for CMAKE_COMPILER_IS_MINGW only after enabling a language when it
might actually be set.  Previously this worked by accident because the
check for working compiler or a second language enabled would cause the
code path to be taken.

Store UsingMinGW as an instance member of cmGlobalNinjaGenerator so that
it is reset on each reconfigure.  Otherwise cmake-gui cannot switch
between build trees for MinGW or non-MinGW tools.