Add debug targets to depends Makefile

Packages can now be built individually and for each stage. This allows
easier debugging.

contrib/depends/Makefile

@@ -217,4 +217,6 @@ download-win:
 	@$(MAKE) -s HOST=x86_64-w64-mingw32 download-one
 download: download-osx download-linux download-win
 
+ $(foreach package,$(all_packages),$(eval $(call ext_add_stages,$(package))))
+
 .PHONY: install cached download-one download-osx download-linux download-win download check-packages check-sources

contrib/depends/funcs.mk

@@ -213,6 +213,14 @@ $(1): | $($(1)_cached_checksum)
 
 endef
 
+stages = fetched extracted preprocessed configured built staged postprocessed cached cached_checksum
+
+define ext_add_stages
+$(foreach stage,$(stages),
+          $(1)_$(stage): $($(1)_$(stage))
+          .PHONY: $(1)_$(stage))
+endef
+
 # These functions create the build targets for each package. They must be
 # broken down into small steps so that each part is done for all packages
 # before moving on to the next step. Otherwise, a package's info