title: including protobuf3 headers in Qt project on Mac
body: <p>as the title indicates, I am trying to compile a simple project in Qt that makes use of the addressbook.pb.h/addressbook.pb.cc files that are generated in the google protobuf3 tutorial for C++ users. Here is my .pro file</p>
  
  <pre><code>#-------------------------------------------------
  #
  # Project created by QtCreator 2016-11-07T10:33:12
  #
  #-------------------------------------------------
  
  QT       += core gui
  
  greaterThan(QT_MAJOR_VERSION, 4): QT += widgets
  
  TARGET = prototest
  TEMPLATE = app
  
  
  SOURCES += main.cpp\
          mainwindow.cpp \
      ../protobuf/examples/addressbook.pb.cc
  
  HEADERS  += mainwindow.h \
      ../protobuf/examples/addressbook.pb.h
  
  FORMS    += mainwindow.ui
  
  INCLUDEPATH += /usr/local/Cellar/protobuf/3.1.0/include
  
  LIBS += /usr/local/Cellar/protobuf/3.1.0
  </code></pre>
  
  <p>Unfortunately, this throws</p>
  
  <pre><code>:-1: error: can't map file, errno=22 file '/usr/local/Cellar/protobuf/3.1.0' for architecture x86_64
  </code></pre>
  
  <p>I'm stuck at this point. Any help would be greatly appreciated. I have been able to compile and run the exact same (addressbook.*) files using Xcode8. </p>