[BUILD] ensure that makefile understands USE_DLMALLOC=1

USE_DLMALLOC=1 was ignored since last makefile update. It's better
to keep it running for existing setups.
diff --git a/Makefile b/Makefile
index 43c05fd..4f3a723 100644
--- a/Makefile
+++ b/Makefile
@@ -6,6 +6,7 @@
 # Valid USE_* options are the following. Most of them are automatically set by
 # the TARGET, others have to be explictly specified :
 #   USE_CTTPROXY         : enable CTTPROXY on Linux (needs kernel patch).
+#   USE_DLMALLOC         : enable use of dlmalloc (see DLMALLOC_SRC)
 #   USE_EPOLL            : enable epoll() on Linux 2.6. Automatic.
 #   USE_EPOLL_WORKAROUND : enable epoll() bug workaround. Automatic.
 #   USE_GETSOCKNAME      : enable getsockname() on Linux 2.2. Automatic.
@@ -346,6 +347,13 @@
 BUILD_OPTIONS  += $(call ignore_implicit,USE_REGPARM)
 endif
 
+ifneq ($(USE_DLMALLOC),)
+BUILD_OPTIONS  += $(call ignore_implicit,USE_DLMALLOC)
+ifeq ($(DLMALLOC_SRC),)
+DLMALLOC_SRC=src/dlmalloc.c
+endif
+endif
+
 ifneq ($(DLMALLOC_SRC),)
 # May be changed to match PAGE_SIZE on every platform
 DLMALLOC_THRES = 4096