fix retrieval of PYTHON_SITE_PACKAGES_DIR - broke debian builds

Author: jef-n

cmake/FindPythonLibrary.cmake

@@ -26,10 +26,10 @@
 
 INCLUDE(CMakeFindFrameworks)
 
-if(EXISTS "${PYTHON_INCLUDE_PATH}" AND EXISTS "${PYTHON_LIBRARY}")
+if(EXISTS "${PYTHON_INCLUDE_PATH}" AND EXISTS "${PYTHON_LIBRARY}" AND EXISTS "${PYTHON_SITE_PACKAGES_DIR}")
   # Already in cache, be silent
   set(PYTHONLIBRARY_FOUND TRUE)
-else(EXISTS "${PYTHON_INCLUDE_PATH}" AND EXISTS "${PYTHON_LIBRARY}")
+else(EXISTS "${PYTHON_INCLUDE_PATH}" AND EXISTS "${PYTHON_LIBRARY}" AND EXISTS "${PYTHON_SITE_PACKAGES_DIR}")
 
   set(_custom_python_fw FALSE)
   if(APPLE AND PYTHON_CUSTOM_FRAMEWORK)
@@ -110,4 +110,4 @@ else(EXISTS "${PYTHON_INCLUDE_PATH}" AND EXISTS "${PYTHON_LIBRARY}")
     endif(PYTHONLIBRARY_FIND_REQUIRED)
   endif(PYTHONLIBRARY_FOUND)
 
-endif (EXISTS "${PYTHON_INCLUDE_PATH}" AND EXISTS "${PYTHON_LIBRARY}")
+endif (EXISTS "${PYTHON_INCLUDE_PATH}" AND EXISTS "${PYTHON_LIBRARY}" AND EXISTS "${PYTHON_SITE_PACKAGES_DIR}")