title: Multicore compilation with Qt 5.1 and jom on Windows 7 failed
body: <p>I have installed Qt 5.1.1 with MinGW 4.8 and QtCreator through Qt Online installer.
  I launched Qt creator and made new project (Qt application Desktop) with MainWindow class based on QMainWindow (default). It works fine and popped up a mainwindow after compilation.</p>
  
  <p>However, when I used C:\Qt\Qt5.1.1\Tools\QtCreator\bin\jom.exe instead of C:\Qt\Qt5.1.1\Tools\mingw48_32\bin\mingw32-make.exe , I got some output like below.</p>
  
  <pre><code>jom 1.0.13 - empower your cores
  
  C:\Qt\Qt5.1.1\Tools\QtCreator\bin\jom.exe -f Makefile.Release
  g++ -c -pipe -fno-keep-inline-dllexport -O2 -frtti -Wall -Wextra -fexceptions -mthreads -DUNICODE -DQT_NO_DEBUG -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -DQT_NEEDS_QMAIN -I../foobar -I'C:/Qt/Qt5.1.1/5.1.1/mingw48_32/include' -I'C:/Qt/Qt5.1.1/5.1.1/mingw48_32/include/QtWidgets' -I'C:/Qt/Qt5.1.1/5.1.1/mingw48_32/include/QtGui' -I'C:/Qt/Qt5.1.1/5.1.1/mingw48_32/include/QtCore' -I'release' -I'.' -I'.' -I'C:/Qt/Qt5.1.1/5.1.1/mingw48_32/mkspecs/win32-g++' -o release/main.o ../foobar/main.cpp
  In file included from ../foobar/main.cpp:1:0:
  ../foobar/mainwindow.h:4:23: fatal error: QMainWindow: No such file or directory
   #include &lt;QMainWindow&gt;
                         ^
  compilation terminated.
  jom: D:\work\build-foobar-Desktop_Qt_5_1_1_MinGW_32bit-Release\Makefile.Release [release\main.o] Error 1
  jom: D:\work\build-foobar-Desktop_Qt_5_1_1_MinGW_32bit-Release\Makefile [release] Error 2
  15:17:38: Process "C:\Qt\Qt5.1.1\Tools\QtCreator\bin\jom.exe" stopped with code 2
  Error while building/deploying project foobar (kit: Desktop Qt 5.1.1 MinGW 32bit)
  During step 'Make'
  15:17:38: Elapsed time: 00:00.
  </code></pre>
  
  <p>My .pro is like this.</p>
  
  <pre><code>#-------------------------------------------------
  #
  # Project created by QtCreator 2013-09-08T15:16:13
  #
  #-------------------------------------------------
  QT       += core gui
  greaterThan(QT_MAJOR_VERSION, 4): QT += widgets
  TARGET = foobar
  TEMPLATE = app
  SOURCES += main.cpp\ mainwindow.cpp
  HEADERS  += mainwindow.h
  FORMS    += mainwindow.ui
  </code></pre>
  
  <p>I think the line in .pro <code>greaterThan(QT_MAJOR_VERSION, 4): QT += widgets</code> and in the output during compilation <code>-I'C:/Qt/Qt5.1.1/5.1.1/mingw48_32/include/QtWidgets'</code> looks fine, but failed.</p>
  
  <p>How can I compile with Qt 5.1 and jom on Windows 7 (32 bit)?</p>
  
  <p>Thanks in advance.</p>