build: retry-on-host-error while downloading files

* We might as well "try as hard as we can" to get build files.
* With this setting, DNS resolution errors count as retryable
  errors.  wget will retry to get files up to 20 times by default.
* Should improve build relability on less-than-ideal network
  connections. (aka, a brief network disconnection shouldn't stop
  a local build if possible)

Change-Id: I39652fafb5eee237c463e1b777b1057ee1d53db1

build/jam/FileRules

@@ -461,7 +461,7 @@ if $(HAIKU_NO_DOWNLOADS) = 1 {
 	actions DownloadLocatedFile1
 	{
 		source="$(2)"
-		wget --retry-connrefused --timeout 30 -O "$(1)" $(URL) || exit 1
+		wget --retry-connrefused --retry-on-host-error --timeout 30 -O "$(1)" $(URL) || exit 1
 		touch "$(1)"
 	}
 }