* common.mk: add comments how to use "make benchmark"

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@52176 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ChangeLog

@@ -1,3 +1,7 @@
+Sun Oct 18 12:59:22 2015  KOSAKI Motohiro  <kosaki.motohiro@gmail.com>
+
+	* common.mk: add comments how to use "make benchmark"
+
 Sun Oct 18 12:58:15 2015  Tanaka Akira  <akr@fsij.org>
 
 	* lib/securerandom.rb: Specify frozen_string_literal: true.

common.mk

@@ -893,6 +893,11 @@ COMPARE_RUBY = $(BASERUBY)
 ITEM =
 OPTS =
 
+# You can pass several options through OPTS environmanent variable.
+# $ make benchmark OPTS="--help" displays more dtail.
+# for example,
+#  $ make benchmark COMPARE_RUBY="ruby-trunk" OPTS="-e ruby-2.2.2"
+# This command compares trunk and built-ruby and 2.2.2
 benchmark: $(PROGRAM) PHONY
 	$(BASERUBY) $(srcdir)/benchmark/driver.rb -v \
 	            --executables="$(COMPARE_RUBY); built-ruby::$(RUNRUBY)" \