ExternalProject: Use default file name if extracting from URL fails

The download/extract step uses the file name only internally so we can
just use a fallback default name if one cannot be extracted from the
URL.

Modules/ExternalProject.cmake

@@ -1878,7 +1878,12 @@ function(_ep_add_download_command name)
         elseif(no_extract)
           get_filename_component(fname "${fname}" NAME)
         else()
-          message(FATAL_ERROR "Could not extract tarball filename from url:\n  ${url}")
+          # Fall back to a default file name.  The actual file name does not
+          # matter because it is used only internally and our extraction tool
+          # inspects the file content directly.  If it turns out the wrong URL
+          # was given that will be revealed during the build which is an easier
+          # place for users to diagnose than an error here anyway.
+          set(fname "archive.tar")
         endif()
         string(REPLACE ";" "-" fname "${fname}")
         set(file ${download_dir}/${fname})