depends: add a fallback path in case package sources go missing

If a source url fails to download, try again at
$FALLBACK_DOWNLOAD_PATH/file.name, where FALLBACK_DOWNLOAD_PATH can be
overridden by the user.

depends/Makefile

@@ -6,6 +6,7 @@ SDK_PATH ?= $(BASEDIR)/SDKs
 NO_QT ?=
 NO_WALLET ?=
 NO_UPNP ?=
+FALLBACK_DOWNLOAD_PATH ?= https://bitcoincore.org/depends-sources
 
 BUILD = $(shell ./config.guess)
 HOST ?= $(BUILD)