- make 'install' DESTDIR-aware

--- cmake/SetupSubPyInstall.cmake.in.orig	2021-10-09 05:02:12 UTC
+++ cmake/SetupSubPyInstall.cmake.in
@@ -9,6 +9,7 @@ message("  CMAKE_INSTALL_PREFIX: ${CMAKE_INSTALL_PREFI
 message("  CMAKE_CURRENT_BINARY_DIR: ${CMAKE_CURRENT_BINARY_DIR}")
 execute_process(COMMAND ${PYTHON_EXECUTABLE} setup_sub.py install
                         "--prefix=${CMAKE_INSTALL_PREFIX}"
+                        "--root=$ENV{DESTDIR}"
                 WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
                 RESULT_VARIABLE res_var)
 if(NOT "${res_var}" STREQUAL "0")