wrapping some makefile vars in ifndef's

Author: Vlad Ionescu

Makefile

@@ -2,10 +2,18 @@ ifndef TMPDIR
 TMPDIR := /tmp
 endif
 
-RABBITMQ_NODENAME=rabbit
-RABBITMQ_SERVER_START_ARGS=
-RABBITMQ_MNESIA_DIR=$(TMPDIR)/rabbitmq-$(RABBITMQ_NODENAME)-mnesia
-RABBITMQ_LOG_BASE=$(TMPDIR)
+ifndef RABBITMQ_NODENAME
+RABBITMQ_NODENAME:=rabbit
+endif
+ifndef RABBITMQ_SERVER_START_ARGS
+RABBITMQ_SERVER_START_ARGS:=
+endif
+ifndef RABBITMQ_MNESIA_DIR
+RABBITMQ_MNESIA_DIR:=$(TMPDIR)/rabbitmq-$(RABBITMQ_NODENAME)-mnesia
+endif
+ifndef RABBITMQ_LOG_BASE
+RABBITMQ_LOG_BASE:=$(TMPDIR)
+endif
 
 SOURCE_DIR=src
 EBIN_DIR=ebin