add gb_ExternalProject_use_external to deal with ExternalProject inter-dep

One may have multiple ExternalProject in a given module
and these ExternalProject may have dependencies among each others
This api allow to explicit such dependencies so that the
ExternalProjects in a given module are built in the right order

Change-Id: Ib8a1b9bdcad0dda08b6fe133113b01a80e02421c

solenv/gbuild/ExternalProject.mk

@@ -103,4 +103,10 @@ $(foreach target,$(2),$(call gb_ExternalProject_register_target,$(1),$(target)))
 
 endef
 
+# Make an external Project depend on another external project
+define gb_ExternalProject_use_external
+$(call gb_ExternalProject_get_preparation_target,$(1)) : $(call gb_ExternalProject_get_target,$(2))
+
+endef
+
 # vim: set noet sw=4 ts=4: