Order ext/configure-ext.mk after prepare-gems

ext/configure-ext.mk is generated by template/configure-ext.mk.tmpl,
which globs `.bundle/gems/**/extconf.rb` to enumerate bundled-gem
extensions. On a clean tree, `make html` regenerated configure-ext.mk
before prepare-gems extracted the bundled gems, so the glob found
nothing and exts.mk lacked rules for bundled-gem extensions. rbs's
extension never compiled, and RubyGems then refused to activate
rbs-4.0.2 as a runtime dependency of rdoc-7.2.0 with "Ignoring
rbs-4.0.2 because its extensions are not built", aborting
tool/rdoc-srcdir.

Add prepare-gems to configure-ext.mk's prerequisites so the bundled
gem directories exist before the template runs.

https://claude.ai/code/session_01YRoRyZPew2LtN6u6BX8bzx

Author: claude

common.mk

@@ -346,7 +346,8 @@ $(EXTS_MK): ext/configure-ext.mk $(srcdir)/template/exts.mk.tmpl \
 	    $(srcdir)/template/exts.mk.tmpl --gnumake=$(gnumake) --configure-exts=ext/configure-ext.mk
 
 ext/configure-ext.mk: $(PREP) all-incs $(MKFILES) $(RBCONFIG) $(LIBRUBY) \
-		$(srcdir)/template/configure-ext.mk.tmpl update-default-gemspecs
+		$(srcdir)/template/configure-ext.mk.tmpl update-default-gemspecs \
+		$(HAVE_BASERUBY:yes=prepare-gems)
 	$(ECHO) generating makefiles $@
 	$(Q)$(MAKEDIRS) $(@D)
 	$(Q)$(MINIRUBY) $(tooldir)/generic_erb.rb -o $@ -c \