title: "no such sysroot directory" while building qt project
body: <p>MacOS <strong>Mojave</strong> Version 10.14 (18A389).</p>
  
  <p>Today I updated Xcode to version 10.0 (10A255). End when I try to build my Qt project I get an error: </p>
  
  <pre><code>clang: warning: no such sysroot directory: '/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk' [-Wmissing-sysroot]
  In file included from ../greed/timediagram.cpp:1:
  In file included from ../../../Qt/5.9.1/clang_64/lib/QtGui.framework/Headers/QtGui:3:
  In file included from /Users/arsenyspiridonov/Qt/5.9.1/clang_64/lib/QtGui.framework/Headers/QtGuiDepends:3:
  In file included from /Users/arsenyspiridonov/Qt/5.9.1/clang_64/lib/QtCore.framework/Headers/QtCore:4:
  In file included from ../../../Qt/5.9.1/clang_64/lib/QtCore.framework/Headers/qglobal.h:47:
  In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/utility:202:
  In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/cstring:61:
  /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/string.h:61:15: fatal error: 'string.h' file not found
  #include_next &lt;string.h&gt;
                ^~~~~~~~~~
  1 error generated.
  make: *** [timediagram.o] Error 1
  16:13:47: Процесс «/usr/bin/make» завершился с кодом 2.
  Ошибка при сборке/установке проекта greed (комплект: Desktop Qt 5.9.1 clang 64bit)
  Во время выполнения этапа «Сборка»
  </code></pre>
  
  <p>What's the problem? How to fix it?</p>