Define make sync-default-gems

to run tool/sync_default_gems.rb

common.mk

@@ -1289,6 +1289,10 @@ yes-test-bundler: yes-test-bundler-prepare
 		--require spec_helper $(RSPECOPTS) spec/bundler/$(BUNDLER_SPECS)
 no-test-bundler:
 
+GEM = up
+sync-default-gems:
+	$(Q) $(XRUBY) -C "$(srcdir)" tool/sync_default_gems.rb $(GEM)
+
 UNICODE_FILES = $(UNICODE_SRC_DATA_DIR)/UnicodeData.txt \
 		$(UNICODE_SRC_DATA_DIR)/CompositionExclusions.txt \
 		$(UNICODE_SRC_DATA_DIR)/NormalizationTest.txt \
@@ -1488,6 +1492,7 @@ help: PHONY
 	"  test-rubyspec:       same as test-spec" \
 	"  test-bundler:        run the Bundler spec" \
 	"  test-bundled-gems:   run the test suite of bundled gems" \
+	"  sync-default-gems:   sync default gems from upstream [GEM=<gem_name>]" \
 	"  up:                  update local copy and autogenerated files" \
 	"  benchmark:           benchmark this ruby and COMPARE_RUBY." \
 	"  gcbench:             gc benchmark [GCBENCH_ITEM=<item_name>]" \