Merge bitcoin#12715: depends: Add 'make clean' rule

aff16fd depends: Add 'make clean' and 'make clean-all' rules (Henrik Jonsson)

Pull request description:

  It's useful to have a standard way to clean up the work done by the
  depends system when testing changes to it.

Tree-SHA512: d56686ca810c2531d852049420f242178d124d45a2508feeac0acf80408e8a95fc9915fb777ff0cbc4edc496990f5fdecf699c87d7986164c74b8fbf418ebf49

depends/Makefile

@@ -1,6 +1,7 @@
 .NOTPARALLEL :
 
 SOURCES_PATH ?= $(BASEDIR)/sources
+WORK_PATH = $(BASEDIR)/work
 BASE_CACHE ?= $(BASEDIR)/built
 SDK_PATH ?= $(BASEDIR)/SDKs
 NO_QT ?=
@@ -28,9 +29,9 @@ else
 release_type=release
 endif
 
-base_build_dir=$(BASEDIR)/work/build
-base_staging_dir=$(BASEDIR)/work/staging
-base_download_dir=$(BASEDIR)/work/download
+base_build_dir=$(WORK_PATH)/build
+base_staging_dir=$(WORK_PATH)/staging
+base_download_dir=$(WORK_PATH)/download
 canonical_host:=$(shell ./config.sub $(HOST))
 build:=$(shell ./config.sub $(BUILD))
 
@@ -182,4 +183,4 @@ download-win:
 	@$(MAKE) -s HOST=x86_64-w64-mingw32 download-one
 download: download-osx download-linux download-win
 
-.PHONY: install cached download-one download-osx download-linux download-win download check-packages check-sources
+.PHONY: install cached clean clean-all download-one download-osx download-linux download-win download check-packages check-sources