Fix bad add_custom_command() in distclean target

`add_custom_command()` has 2 formats: OUTPUT and TARGET. This is TARGET,
but TARGET does not permit OUTPUT.

see https://cmake.org/cmake/help/v3.31/policy/CMP0175.html
and https://cmake.org/cmake/help/v3.31/command/add_custom_command.html

Also, I converted to now-prefered lowercase (only lowercase works in my
IDE to pull up context-sensitive help for CMake). And I replaced a Make-
oriented COMMAND on `add_custom_target()` to a proper `COMMENT`.

cmake/modules/TargetDistclean.cmake

@@ -3,9 +3,11 @@
 # Jan Woetzel 04/2003
 #
 
-IF (UNIX)
-  ADD_CUSTOM_TARGET (distclean @echo cleaning for source distribution)
-  SET(DISTCLEANED
+if(UNIX)
+  add_custom_target(distclean
+     COMMENT "cleaning for source distribution"
+  )
+  set(DISTCLEANED
    cmake.depends
    cmake.check_depends
    CMakeCache.txt
@@ -16,13 +18,17 @@ IF (UNIX)
    gmon.out
    *~
   )
-  
-  ADD_CUSTOM_COMMAND(
-    DEPENDS clean
+  add_custom_command(
+     COMMENT "running target clean"
+     TARGET distclean
+     COMMAND ${CMAKE_COMMAND} --build ${CMAKE_BINARY_DIR} --target clean
+  )
+  add_custom_command(
+    POST_BUILD
     COMMENT "distribution clean"
     COMMAND rm
     ARGS    -Rf CMakeTmp ${DISTCLEANED}
     TARGET  distclean
   )
-ENDIF(UNIX)
+endif(UNIX)