title: CMake Qt5 can't generate header file for ui file AUTOUIC
body: <p>I can't generate the header for my ui files with cmake 3.5.2 and Qt 5.9.</p>
  
  <p>My CMakeFileLists.txt:</p>
  
  <pre><code>cmake_minimum_required(VERSION 3.5)
  project( fc_app )
  message( STATUS "Configuring project")
  
  
  set(CMAKE_AUTOMOC ON)
  set(CMAKE_AUTOUIC ON)
  set(CMAKE_AUTORCC ON)
  set(CMAKE_INCLUDE_CURRENT_DIR ON)
  
  message( STATUS "search OpenCV")
  find_package(
      OpenCV
      3.2.0
      REQUIRED
  )
  
  message( STATUS "search Qt")
  find_package(
      Qt5
      5.5.1
      REQUIRED
          Core
          Gui
          Widgets
          Multimedia
  )
  
  message( STATUS "search Boost")
  find_package(
      Boost
      1.58.0
      REQUIRED
  )
  
  
  file( GLOB_RECURSE source_files src/* )
  file( GLOB_RECURSE header_files include/* )
  file( GLOB_RECURSE ui_files ui/* )
  file( GLOB_RECURSE res_files res/* )
  
  add_executable(
      fc_app
      ${source_files}
      ${header_files}
      ${ui_files}
  )
  
  target_link_libraries(
      fc_app
      ${OpenCV_LIBS}
      Qt5::Widgets
      Qt5::Multimedia
      ${Boost_LIBRARIES}
  )
  </code></pre>
  
  <p>I get the following error (in make) when I run <code>cmake .. &amp; make</code> in the root/build/ directory. The ui file is a simple QMainWindow with 2 buttons in it, so I don't unterstand why the generation of ui_mainwindow.h failed. I tried to remake the ui file with an other version of Qt5 Designer too.</p>
  
  <pre><code>File '/blablablabla/mainwindow.ui' is not valid
  AUTOUIC: error: process for ui_mainwindow.h needed by
   "/blablablabla/mainwindow.cpp"
  failed:
  File '/blablablabla/mainwindow.ui' is not valid
  </code></pre>