title: Error when compiling blender for windows - OSL not found
body: <p>I am trying to compile blender for windows. I have followed the instructions on the <a href="https://wiki.blender.org/index.php/Dev:Doc/Building_Blender/Windows" rel="nofollow noreferrer">blender wiki</a></p>
  
  <p>When I run the the command "make full" in the command prompt I get the following error:</p>
  
  <pre><code>-- 64 bit compiler detected.
  -- Visual Studio 2013 detected.
  CMake Warning at build_files/cmake/platform/platform_win32.cmake:436 (message):
    LLVM debug libs not present on this system.  Using release libs for debug
    builds.
  Call Stack (most recent call first):
    CMakeLists.txt:931 (include)
  
  
  -- OSL not found 
  CMake Error at CMakeLists.txt:1573 (message):
    Missing:
    "E:/blender-git/blender/../lib/win64_vc12/python/include/python3.6/Python.h",
  
  
    Set the cache entry 'PYTHON_INCLUDE_DIR' to point to a valid python include
    path.  Containing Python.h for python version "3.6"
  
  
  -- Configuring incomplete, errors occurred!
  See also "E:/blender-git/build_windows_Full_x64_vc12_Release/CMakeFiles/CMakeOutput.log".
  Microsoft (R) Build Engine version 12.0.40629.0
  [Microsoft .NET Framework, version 4.0.30319.42000]
  Copyright (C) Microsoft Corporation. All rights reserved.
  
  MSBUILD : error MSB1009: Project file does not exist.
  Switch: E:\blender-git\blender\..\build_windows_Full_x64_vc12_Release\Blender.sln
  "Build Failed"
  </code></pre>
  
  <p>It seems to me that CMake can't find the needed python files. I have Python 3.6 installed. It seems that I should be able to change the value of 'PYTHON_INCLUDE_DIR' to point to the python installation folder, but there is no hint in which file this should be changed.  </p>