build: Update uninstall.cmake.in

The 'exec_program' command is deprecated in a newer CMake, resulting
in the below warning for each uninstalled file.

CMake Warning (dev) at uninstall.cmake:17 (exec_program):
  Policy CMP0153 is not set: The exec_program command should not be called.
  Run "cmake --help-policy CMP0153" for policy details.  Use the cmake_policy
  command to set the policy and suppress this warning.

  Use execute_process() instead.
This warning is for project developers.  Use -Wno-dev to suppress it.

uninstall.cmake.in

@@ -14,10 +14,10 @@ string(REGEX REPLACE "\n" ";" files "${files}")
 foreach(file ${files})
   message(STATUS "Uninstalling $ENV{DESTDIR}${file}")
   if(IS_SYMLINK "$ENV{DESTDIR}${file}" OR EXISTS "$ENV{DESTDIR}${file}")
-    exec_program(
-      "@CMAKE_COMMAND@" ARGS "-E remove \"$ENV{DESTDIR}${file}\""
+    execute_process(
+      COMMAND "@CMAKE_COMMAND@" -E remove "$ENV{DESTDIR}${file}"
       OUTPUT_VARIABLE rm_out
-      RETURN_VALUE rm_retval
+      RESULT_VARIABLE rm_retval
     )
     if(NOT "${rm_retval}" STREQUAL 0)
       message(FATAL_ERROR "Problem when removing $ENV{DESTDIR}${file}")