ExternalProject: Catch empty REMOTE or LOCAL earlier

If we are given an empty string for URL, or we have a logic error that leads
to the file we download to being an empty string, we will now catch this at
CMake configure time instead of whenever the download is attempted at
build time.

Modules/ExternalProject.cmake

@@ -1518,6 +1518,13 @@ function(_ep_write_downloadfile_script
   netrc
   netrc_file
 )
+  if("x${REMOTE}" STREQUAL "x")
+    message(FATAL_ERROR "REMOTE can't be empty")
+  endif()
+  if("x${LOCAL}" STREQUAL "x")
+    message(FATAL_ERROR "LOCAL can't be empty")
+  endif()
+
   if(timeout)
     set(TIMEOUT_ARGS TIMEOUT ${timeout})
     set(TIMEOUT_MSG "${timeout} seconds")

Modules/ExternalProject/download.cmake.in

@@ -71,14 +71,6 @@ function(sleep_before_download attempt)
   execute_process(COMMAND "${CMAKE_COMMAND}" -E sleep "${sleep_seconds}")
 endfunction()
 
-if("@LOCAL@" STREQUAL "")
-  message(FATAL_ERROR "LOCAL can't be empty")
-endif()
-
-if("@REMOTE@" STREQUAL "")
-  message(FATAL_ERROR "REMOTE can't be empty")
-endif()
-
 if(EXISTS "@LOCAL@")
   check_file_hash(has_hash hash_is_good)
   if(has_hash)