Make warnings if objdir and targetdir point to the same directory

As the title says, if objdir and targetdir point to the same directory, then we'll get some errors:

Basic.C.make:72: warning: overriding commands for target `.'
Basic.C.make:64: warning: ignoring old commands for target `.'

This makes sense if we look at the makefile:

$(TARGETDIR):
	@echo Creating $(TARGETDIR)
ifeq (posix,$(SHELLTYPE))
	$(SILENT) mkdir -p $(TARGETDIR)
else
	$(SILENT) mkdir $(subst /,\\,$(TARGETDIR))
endif

...

$(OBJDIR):
	@echo Creating $(OBJDIR)
ifeq (posix,$(SHELLTYPE))
	$(SILENT) mkdir -p $(OBJDIR)
else
	$(SILENT) mkdir $(subst /,\\,$(OBJDIR))
endif

If the directories are the same, then only one of these targets needs to be generated.

Start by locating the generated Makefile rules for TARGETDIR and OBJDIR shown in the issue. Confirm how generation behaves when both variables resolve to the same directory, then verify that only one directory target rule is emitted and the duplicate-target warnings disappear.