Switch calls to find_package(PythonInterp) and find_package(PythonLib…

…s) to avoid linking with wrong python

The cmake documentation
(https://cmake.org/cmake/help/v3.5/module/FindPythonLibs.html) says
that:
"If calling both find_package(PythonInterp) and
find_package(PythonLibs), call find_package(PythonInterp) first to get
the currently active Python version by default with a consistent version
of PYTHON_LIBRARIES."

CMakeLists.txt

@@ -64,8 +64,8 @@ if ( CGAL_FOUND )
       message(STATUS "BUILD_RUBY is set to OFF: no CGAL-bindings for Ruby will be generated.")
     endif()
     if (${BUILD_PYTHON})
-      FIND_PACKAGE(PythonLibs)
       find_package(PythonInterp)
+      FIND_PACKAGE(PythonLibs)
       if (NOT PYTHONLIBS_FOUND)
         SET(BUILD_PYTHON OFF)
         message(WARNING "Python has not been found, CGAL-bindings for Python will not be generated.")