fdo#51115: gb_Extension_add_file: fix Windows build:

The DLL is not actually a make target, but is handled via the AUXTARGETS
mechanism in LinkTarget.mk; add an ugly hack here to hopefully make
adding libraries to extensions work.

Change-Id: I85ee891fe8b7d470e09d9cb1d5a48d8b11b50d6e

solenv/gbuild/Extension.mk

@@ -136,12 +136,15 @@ endef
 
 # adding a file creates a dependency to it
 # file is copied to $(WORKDIR)
+# $(3) is the target of the copied file, and $(4) can be used to override that
+# with a different actual file, which is needed in gb_Extension_add_library
+# to make it work on Windows where the DLL doesn't have a gbuild target...
 define gb_Extension_add_file
 $(call gb_Extension_get_target,$(1)) : FILES += $(2)
 $(call gb_Extension_get_target,$(1)) : $(call gb_Extension_get_rootdir,$(1))/$(2)
 $(call gb_Extension_get_rootdir,$(1))/$(2) : $(3)
 	mkdir -p $$(dir $$@) && \
-	cp -f $$< $$@
+	cp -f $(if $(4),$(4),$(3)) $$@
 
 endef
 
@@ -159,6 +162,7 @@ endef
 # add a library from the solver; DO NOT use gb_Library_get_target
 define gb_Extension_add_library
 $(call gb_Extension_add_file,$(1),$(call gb_Library_get_runtime_filename,$(2)),\
+	$(call gb_Library_get_target,$(2)),\
 	$(gb_Helper_OUTDIRLIBDIR)/$(call gb_Library_get_runtime_filename,$(2)))
 endef
 
@@ -169,6 +173,7 @@ endef
 # add an executable from the solver
 define gb_Extension_add_executable
 $(call gb_Extension_add_file,$(1),$(notdir $(call gb_Executable_get_target,$(2))),\
+	$(call gb_Executable_get_target,$(2)),\
 	$(call gb_Executable_get_target,$(2)))
 endef