gbuild: ExternalProject: always build externals from scratch

It could happen that externals were built incrementally, in case
something the ExternalProject depends on was updated.  To prevent this,
change the dependencies so that the UnpackedTarball is unpacked again
when the ExternalProject's dependencies are newer.  This is possible
without introducing a new target for the purpose due to the refactorings
in previous commits that enforce the name of UnpackedTarball.

Change-Id: Ie7a84064ec2ffc76175cd2b7792517e68664a461

solenv/gbuild/ExternalProject.mk

@@ -19,6 +19,19 @@
 # Package is sufficient if no files--e.g., headers--from the unpacked
 # tarball need to be delivered.)
 #
+# An ExternalProject always uses one UnpackedTarball with the same name.
+# The dependency strucure ensures that any change on a dependency
+# of the ExternalProject will cause the UnpackedTarball to be unpacked
+# again, so the ExternalProject always does a clean build and is not at
+# the mercy of the external's build system's dubious incremental builds.
+#
+# ExternalProject target
+# => ExternalProject state target(s) (these actually build stuff)
+#    => UnpackedTarball target (unpack the tarball)
+#       => UnpackedTarball prepare target
+#          => ExternalProject prepare target
+#             => stuff the external depends upon
+#
 # ExternalProject has no gbuild abstraction for actually building the
 # external code, so it is necessary to define rule(s) and recipe(s) to
 # handle it. It does not matter if there are several rules handling
@@ -62,9 +75,9 @@ define gb_ExternalProject_ExternalProject
 $(call gb_ExternalProject_get_target,$(1)) : EXTERNAL_WORKDIR := $(call gb_UnpackedTarball_get_dir,$(1))
 
 $(call gb_ExternalProject_get_preparation_target,$(1)) :| $(dir $(call gb_ExternalProject_get_target,$(1))).dir
-$(call gb_ExternalProject_get_preparation_target,$(1)) : $(call gb_UnpackedTarball_get_target,$(1))
+$(call gb_UnpackedTarball_get_preparation_target,$(1)) : $(call gb_ExternalProject_get_preparation_target,$(1))
 $(call gb_ExternalProject_get_clean_target,$(1)) : $(call gb_UnpackedTarball_get_clean_target,$(1))
-$(call gb_ExternalProject_get_target,$(1)) : $(call gb_ExternalProject_get_preparation_target,$(1))
+$(call gb_ExternalProject_get_target,$(1)) : $(call gb_UnpackedTarball_get_target,$(1))
 $(call gb_ExternalProject_get_target,$(1)) :| $(dir $(call gb_ExternalProject_get_target,$(1))).dir
 
 $$(eval $$(call gb_Module_register_target,$(call gb_ExternalProject_get_target,$(1)),$(call gb_ExternalProject_get_clean_target,$(1))))
@@ -87,7 +100,7 @@ endef
 # gb_ExternalProject_register_target project target
 define gb_ExternalProject_register_target
 $(call gb_ExternalProject_get_target,$(1)) : $(call gb_ExternalProject_get_state_target,$(1),$(2))
-$(call gb_ExternalProject_get_state_target,$(1),$(2)) : $(call gb_ExternalProject_get_preparation_target,$(1))
+$(call gb_ExternalProject_get_state_target,$(1),$(2)) : $(call gb_UnpackedTarball_get_target,$(1))
 $(call gb_ExternalProject_get_state_target,$(1),$(2)) :| $(dir $(call gb_ExternalProject_get_state_target,$(1),$(2))).dir
 
 endef