Improve build_qt_with_openssl.cmake to check that generator bitness matches requested one

[jcfr]

The idea would be to edit build_qt_with_openssl.cmake and add code like the one posted below to notify the developer early if the wrong Visual Studio command line prompt has been used.

if(BITS EQUAL 64 AND NOT CMAKE_SIZEOF_VOID_P EQUAL 8)
  message(FATAL_ERROR "Requested bitness does NOT match select generator and/or compiler.")
endif()