SSE defines check build error for some Mac hardware configurations

On some Mac hardware configurations, setting '-no-ssse3' or similar < sse4.x negative option, cascades up and turns off all greater sse* options.

However, there are define checks in src/corelib/tools/qstring.cpp mergeQuestionMarks() that only check for _SSE4_x_, ignoring whether the associated option was turned off via a command line configure option.

See https://github.com/Homebrew/homebrew/issues/28859#issuecomment-41865873 , where -no-ssse3 configure option is used, which should turn off SSE4 code, but _SSE4_x_ code is still executed.

The attached patch ensures the QT_HAVE_SSE4_x defines are honored.