ExternalProject: Fix misuse of IS_NEWER_THAN in timestamp checks

When using a file system which only has second resolution timestamps,
there is a reasonably high likelihood of timestamps being the same.
The IS_NEWER_THAN test returns true when timestamps are the same,
so don't redo downloads when they match exactly.

Modules/ExternalProject/gitclone.cmake.in

@@ -7,7 +7,8 @@ set(quiet "@quiet@")
 set(script_dir "@CMAKE_CURRENT_FUNCTION_LIST_DIR@/ExternalProject")
 include(${script_dir}/captured_process_setup.cmake)
 
-if(NOT "@gitclone_infofile@" IS_NEWER_THAN "@gitclone_stampfile@")
+if(EXISTS "@gitclone_stampfile@" AND EXISTS "@gitclone_infofile@" AND
+   "@gitclone_stampfile@" IS_NEWER_THAN "@gitclone_infofile@")
   if(NOT quiet)
     message(STATUS
       "Avoiding repeated git clone, stamp file is up to date: "

Modules/ExternalProject/hgclone.cmake.in

@@ -7,7 +7,8 @@ set(quiet "@quiet@")
 set(script_dir "@CMAKE_CURRENT_FUNCTION_LIST_DIR@/ExternalProject")
 include(${script_dir}/captured_process_setup.cmake)
 
-if(NOT "@hgclone_infofile@" IS_NEWER_THAN "@hgclone_stampfile@")
+if(EXISTS "@hgclone_stampfile@" AND EXISTS "@hgclone_infofile@" AND
+   "@hgclone_stampfile@" IS_NEWER_THAN "@hgclone_infofile@")
   if(NOT quiet)
     message(STATUS
       "Avoiding repeated hg clone, stamp file is up to date: "