Update build instructions for Windows.

Use VS2013; remove path to qtrepotools, which only exists
in git checkouts.

Task-number: QTBUG-40421
Change-Id: I6409888920041ca1146ee8025aba7ed8aa9c91ad
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>

doc/src/platforms/windows.qdoc

@@ -863,10 +863,14 @@ Arguments:
         and the Qt folder is called \c qt-5 and located under \c C:\qt:
 
         \code
-        REM Set up Windows SDK for 64bit
-        CALL "C:\Program Files (x86)\MSVC10\VC\vcvarsall.bat" amd64
-        set PATH=c:\qt\qt-5\qtbase\bin;c:\qt\qt-5\qtrepotools\bin;c:\qt\qt-5\gnuwin32\bin;%PATH%
-        set QMAKESPEC=win32-msvc2010
+        REM Set up \Microsoft Visual Studio 2013, where <arch> is \c amd64, \c x86, etc.
+        CALL "C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\vcvarsall.bat" <arch>
+        SET _ROOT=C:\qt\qt-5
+        SET PATH=%_ROOT%\qtbase\bin;%_ROOT%\gnuwin32\bin;%PATH%
+        REM Uncomment the below line when using a git checkout of the source repository
+        REM SET PATH=%_ROOT%\qtrepotools\bin;%PATH%
+        SET QMAKESPEC=win32-msvc2013
+        SET _ROOT=
         \endcode
 
         A desktop link can then be created by specifying the command