Makefile tweak for better subproject handling

Don't include the makefile fragments of subprojects that aren't built.

With this the build fails more reasonably when SUBS is set incorrectly.
diff --git a/Makefile b/Makefile
index c661b33..dee6134 100644
--- a/Makefile
+++ b/Makefile
@@ -30,8 +30,7 @@
 
 -include config.mk
 
-include $(srcdir)/math/Dir.mk
-include $(srcdir)/string/Dir.mk
+$(foreach sub,$(SUBS),$(eval include $(srcdir)/$(sub)/Dir.mk))
 
 # Required targets of subproject foo:
 #   all-foo