Make CMAKE_INSTALL_PYTHONDIR persistent

Fixes #197

bindings/python/CMakeLists.txt

@@ -28,6 +28,9 @@ endif()
 string(REGEX REPLACE "^${PYTHON_PREFIX}/" ""
   CMAKE_INSTALL_PYTHONDIR "${PYTHON_SITE_PACKAGES}"
 )
+set(CMAKE_INSTALL_PYTHONDIR "${CMAKE_INSTALL_PYTHONDIR}"
+  CACHE INTERNAL "" FORCE
+)
 set_target_properties(adios2py PROPERTIES
   OUTPUT_NAME adios2
   LIBRARY_OUTPUT_DIRECTORY ${ADIOS2_BINARY_DIR}/${CMAKE_INSTALL_PYTHONDIR}