ExternalProject: Restore case-insensitive download hash check

Refactoring in commit v3.6.0-rc1~47^2 (ExternalProject: Re-implement
download logic as a dedicated script, 2016-05-19) accidentally made the
download hash check case-sensitive.  The hash comparison is done in hex
strings, so restore case-insensitive comparison since the case of the
hex digits does not matter.

Fixes: #16568

Modules/ExternalProject.cmake

@@ -889,7 +889,7 @@ function(_ep_write_downloadfile_script script_filename REMOTE LOCAL timeout no_p
 
   if("${hash}" MATCHES "${_ep_hash_regex}")
     set(ALGO "${CMAKE_MATCH_1}")
-    set(EXPECT_VALUE "${CMAKE_MATCH_2}")
+    string(TOLOWER "${CMAKE_MATCH_2}" EXPECT_VALUE)
   else()
     set(ALGO "")
     set(EXPECT_VALUE "")