allow PROC to be supplied on the make command line (bug #4570)

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@5966 65c4cc65-6c06-0410-ace0-fbb531ad65f3

Makefile

@@ -33,7 +33,7 @@ endif
 
 ifeq (${OSARCH},Linux)
 ifeq ($(CROSS_COMPILE),)
-PROC=$(shell uname -m)
+PROC?=$(shell uname -m)
 else
 PROC=$(CROSS_PROC)
 endif