common.mk: extract bundled gems before configuring extensions

`template/configure-ext.mk.tmpl` discovers bundled-gem C extensions
via `Dir.glob(".bundle/gems/**/extconf.rb")` when `ext/configure-ext.mk`
is regenerated. Without an explicit dependency, `extract-gems` had no
guarantee of running first, so on a clean build the glob returned
nothing and `build-ext` never saw `rbs`.

As a result, `make html` failed with `Gem::MissingSpecError: Could
not find 'rbs' (>= 4.0.0)` once the rbs-integrated `rdoc` SHA landed
in `gems/bundled_gems` (ruby#16940): the `rbs` gemspec, extension binary,
and `gem.build_complete` marker were all absent from `.bundle/`.

Add `extract-gems` as a prereq of `ext/configure-ext.mk`, gated on
`HAVE_BASERUBY` to match the rest of the bundled-gems pipeline. The
existing per-extension Makefile chain (`all: install` →
`install-so install-rb`) then produces the `.so`, build_complete
marker, and the spec copy in `.bundle/specifications/`.

Fixes the `master` matrix of the ruby/actions
"Make HTML for docs.r-l.o/en/" workflow.

Author: st0012

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=extract-gems)
 	$(ECHO) generating makefiles $@
 	$(Q)$(MAKEDIRS) $(@D)
 	$(Q)$(MINIRUBY) $(tooldir)/generic_erb.rb -o $@ -c \