tests/4 qt: Force the use of -std=c++11

Qt5 now requires this, and if no -std option is passed to clang, it does
not enable C++ 11 features and we get a build failures.

test cases/frameworks/4 qt/meson.build

@@ -1,4 +1,6 @@
-project('qt4 and 5 build test', 'cpp')
+project('qt4 and 5 build test', 'cpp',
+  # Qt5 now requires C++ 11 support
+  default_options : ['cpp_std=c++11'])
 
 qt5_modules = ['Widgets']
 foreach qt : ['qt4', 'qt5']