-RUBY_SITELIB := $(shell ruby -rrbconfig -e "puts Config::CONFIG['sitelibdir']")
-RUBY_SITEARCH := $(shell ruby -rrbconfig -e "puts Config::CONFIG['sitearchdir']")
+RUBY_SITELIB := $(shell $(RUBY) -rrbconfig -e "puts Config::CONFIG['sitelibdir']")
+RUBY_SITEARCH := $(shell $(RUBY) -rrbconfig -e "puts Config::CONFIG['sitearchdir']")