title: How to specify the version of python for building pybind11 module
body: <p>I have setup my pybind11 for building python modules written in c++.
  I have python3.6, and python3.7 installed on my ubuntu system, I installed both <code>python3.6-dev</code> and <code>python3.7-dev</code>. then I setup my <code>cmakelists.txt</code> file for my project as following:</p>
  <pre><code>set(PYTHONVERSION &quot;3.7&quot;)
  # None of the lines below works 
  add_definitions(-DPYBIND11_PYTHON_VERSION=&quot;${PYTHONVERSION}&quot;)
  set(PYBIND11_PYTHON_VERSION ${PYTHONVERSION} CACHE STRING &quot;&quot;)
  set(PYBIND11_PYTHON_VERSION ${PYTHONVERSION}) 
  find_package(Python ${PYTHONVERSION} COMPONENTS Development Interpreter)
  
  set(TARGET proj)
  pybind11_add_module(${TARGET} MODULE proj.cpp)
  target_include_directories(${TARGET} PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})
  if (Python_FOUND)
      target_include_directories(${TARGET} PUBLIC ${Python_INCLUDE_DIRS})
  endif()
  
  </code></pre>
  <p>However, when I build the project it still generates modules for <code>python3.6</code> instead of <code>python3.7</code>. Below is my output from cmake, I found cmake finds <code>python3.6</code> first, where I didn't even call <code>findpython</code>.</p>
  <pre><code>[cmake] Not searching for unused variables given on the command line.
  [cmake] -- The C compiler identification is GNU 9.3.0
  [cmake] -- The CXX compiler identification is GNU 9.3.0
  [cmake] -- Detecting C compiler ABI info
  [cmake] -- Detecting C compiler ABI info - done
  [cmake] -- Check for working C compiler: /usr/bin/gcc-9 - skipped
  [cmake] -- Detecting C compile features
  [cmake] -- Detecting C compile features - done
  [cmake] -- Detecting CXX compiler ABI info
  [cmake] -- Detecting CXX compiler ABI info - done
  [cmake] -- Check for working CXX compiler: /usr/bin/g++-9 - skipped
  [cmake] -- Detecting CXX compile features
  [cmake] -- Detecting CXX compile features - done
  [cmake] -- Conan: Adjusting output directories
  [cmake] -- Conan: Using cmake global configuration
  [cmake] -- Conan: Adjusting default RPATHs Conan policies
  [cmake] -- Conan: Adjusting language standard
  [cmake] -- Current conanbuildinfo.cmake directory: /home/mpnv38/develop/parcel_dim_pywrapper/build
  [cmake] -- Conan: Compiler GCC&gt;=5, checking major version 9
  [cmake] -- Conan: Checking correct version: 9
  [cmake] -- Found PythonInterp: /usr/bin/python (found version &quot;3.6.9&quot;) 
  [cmake] -- Found PythonLibs: /usr/lib/x86_64-linux-gnu/libpython3.6m.so
  [cmake] -- Performing Test HAS_CPP14_FLAG
  [cmake] -- Performing Test HAS_CPP14_FLAG - Success
  [cmake] -- Found Python: /usr/bin/python3.7 (found suitable version &quot;3.7.5&quot;, minimum required is &quot;3.7&quot;) found components: Development Interpreter Development.Module Development.Embed 
  [cmake] -- Configuring done
  [cmake] -- Generating done
  </code></pre>
  <p>Did I miss anything?</p>