[CMake] Use RPATH_CHANGE instead RPATH_SET

RPATH_SET is not available until cmake 3.21.0. Use RPATH_CHANGE instead.

(cherry picked from commit f645069)

Author: rintaro

cmake/modules/SetRPATH.cmake

@@ -4,10 +4,18 @@ function(swift_get_set_rpath_script_file out_var)
   set(${out_var} "${SWIFT_SET_RPATH_SCRIPT_FILE}" PARENT_SCOPE)
 endfunction()
 
-# Actual RPATH_SET operation to the file.
+# Actual RPATH_CHANGE operation to the file.
 function(_swift_set_rpath_impl file new_rpath)
-  # NOTE: RPATH_SET is not documented, and works only for ELF and XCOFF. 
-  file(RPATH_SET FILE "${file}" NEW_RPATH "${new_rpath}")
+  # FIXME: Handle non-ELF files. We can't use RPATH_SET because it's only available CMake 3.21.0
+  execute_process(
+    COMMAND readelf -Wd "${file}"
+    COMMAND grep -Po "R(UN)?PATH.*\\[\\K[^\\]]*"
+    OUTPUT_VARIABLE current_rpath
+  )
+  string(STRIP "${current_rpath}" current_rpath)
+
+  # NOTE: RPATH_CHANGE is not documented, and works only for ELF and XCOFF.
+  file(RPATH_CHANGE FILE "${file}" OLD_RPATH "${current_rpath}" NEW_RPATH "${new_rpath}")
 endfunction()
 
 # For 'cmake -P <scirpt>'.