title: No soucers given to target error when adding sources in pybind11 and C++
body: <p>I am trying to make a library written in C++ work in python, using pybind11.</p>
  
  <p>The compilation and building of the source files works without errors, but the resulting files throw this error when installing with pip.</p>
  
  <pre><code> CMake Error at pybind11/tools/pybind11Tools.cmake:131 (add_library):
        Cannot find source file:
  
          ../project/variables.cpp
  
        Tried extensions .c .C .c++ .cc .cpp .cxx .cu .m .M .mm .h .hh .h++ .hm
        .hpp .hxx .in .txx
      Call Stack (most recent call first):
        CMakeLists.txt:5 (pybind11_add_module)
  
  
      CMake Error at pybind11/tools/pybind11Tools.cmake:131 (add_library):
        No SOURCES given to target: project
      Call Stack (most recent call first):
        CMakeLists.txt:5 (pybind11_add_module)
  </code></pre>
  
  <p>I am following the <a href="https://github.com/pybind/cmake_example" rel="nofollow noreferrer">cmake_example</a> and I am sure that the file exists. In fact, if I delete the file the error occur when running</p>
  
  <pre><code>python setup.py sdist
  </code></pre>
  
  <p>The folder structure is like this:</p>
  
  <pre><code>C++ project root folder
  |-- C++ Project source files
  |-- pybind folder
  |------pybind11 source folder
  |----------pybind.cpp file
  |------pybind11 CmakeLists.txt
  |------pybind11 Manifest.in
  </code></pre>
  
  <p>This is the CMakeLists.txt used by pybind.</p>
  
  <pre><code>cmake_minimum_required(VERSION 2.8.12)
  project(project)
  
  add_subdirectory(pybind11)
  pybind11_add_module(project
  project/pybind.cpp
  ../project/variables.cpp
  ../project/instances.cpp
  .
  .
  .
  )
  </code></pre>
  
  <p>Both the C++ and the pybind source folders are included in the <code>Manifest.in</code> file. Furthermore, ther is no error refering to <code>project/pybind.cpp</code> file.</p>
  
  <p>Also, this method of building the python module worked for me a couple months ago in the same project. I have tried downgradin setuptools, pybind11 and cmake but it does not work. I may be wrong, but I think the only chages I have made have been to add a couple C++ headers to the project and som functions in the <code>pybind.cpp</code>file.</p>