Fix rubyspec-capiext dependency

Not to build the rubyspec-capiext extension libraries again on the
next build after the build all extensions get built, ensure these
extensions are up to date when recursively building from exts.mk.

defs/gmake.mk

@@ -513,10 +513,12 @@ ifneq ($(POSTLINK),)
 endif
 	$(Q) $(RMALL) $@.*
 
-rubyspec-capiext: $(patsubst %.c,$(RUBYSPEC_CAPIEXT)/%.$(DLEXT),$(notdir $(wildcard $(srcdir)/$(RUBYSPEC_CAPIEXT)/*.c)))
+RUBYSPEC_CAPIEXT_SO := $(patsubst %.c,$(RUBYSPEC_CAPIEXT)/%.$(DLEXT),$(notdir $(wildcard $(srcdir)/$(RUBYSPEC_CAPIEXT)/*.c)))
+rubyspec-capiext: $(RUBYSPEC_CAPIEXT_SO)
 	@ $(NULLCMD)
 
 ifeq ($(ENABLE_SHARED),yes)
+ruby: $(if $(LIBRUBY_SO_UPDATE),$(RUBYSPEC_CAPIEXT_SO))
 exts: rubyspec-capiext
 endif