s/ENABLE_RUBY/WITH_RUBY/ for consistency.

This didn't happen in r666490 because the Ruby stuff was developed
in a separate branch.


git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@675857 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/lib/Makefile.am b/lib/Makefile.am
index 32240ae..3cb592c 100644
--- a/lib/Makefile.am
+++ b/lib/Makefile.am
@@ -17,7 +17,7 @@
 SUBDIRS += erl
 endif
 
-if ENABLE_RUBY
+if WITH_RUBY
 SUBDIRS += rb
 endif