Don't set empty CMAKE_TOOLCHAIN_FILE or CMAKE_BUILD_TYPE

Don't set empty CMAKE_TOOLCHAIN_FILE or CMAKE_BUILD_TYPE

The CMake Visual Studio 10 generator generates an include() for
the empty CMAKE_TOOLCHAIN_FILE, causing the errors in the bug.

There may be other remaining errors to cause the Windows CMake build
to fail with that generator, but this patch is an improvement
anyway - there is no point in setting empty strings as values for
those variables.

Task-number: QTBUG-27087
Change-Id: I68cce9e3dce07835db5f42777ac02d440f90f967
Reviewed-by: Brad King <brad.king@kitware.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>