configure.ac: revert #3300. We do not need to define that flag for nvcc. We should instead check _MSC_VER for microsoft visual studio

configure.ac

@@ -48,7 +48,6 @@ case "$target" in
 *-*-mingw*|*-*-cygwin*)
   starpu_windows=yes
   libext=a
-  NVCCFLAGS="-D__MINGW32__"
   AC_DEFINE(STARPU_HAVE_WINDOWS, [], [Define this on windows.])
   ;;
 esac