erlang: Use Automake SUBDIRS instead of a manual forwarding rule

Previously, lib/erl/Makefile.am would forward rules like "all" and
"clean" to the src subdir by manually invoking a submake.  However,
specifying src as a SUBDIR accomplishes this more easily and also
ensures that other rules like "distclean" work.

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@1004510 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/lib/erl/Makefile.am b/lib/erl/Makefile.am
index 18028a7..0722f12 100644
--- a/lib/erl/Makefile.am
+++ b/lib/erl/Makefile.am
@@ -17,13 +17,7 @@
 # under the License.
 #
 
-MODULES = \
-	src
-
-all clean docs:
-	for dir in $(MODULES); do \
-		(cd $$dir; ${MAKE} $@); \
-	done
+SUBDIRS = src
 
 install: all
 	mkdir -p $(DESTDIR)$(ERLANG_INSTALL_LIB_DIR_thrift) ; \