replace = by := when it is constant

git-svn-id: http://svn.eurecom.fr/openair4G/trunk@6204 818b1a75-f10b-46b9-bf7c-635c3b92a50f

targets/Makerules

@@ -12,7 +12,7 @@ OBJS_DIR := $(PWD)/objs
 SVN_REV := $(shell svnversion -n .)
 DATE_REV := $(shell date '+%F %T')
 
-NUM_CORES=$(shell cat /proc/cpuinfo | grep processor | wc -l)
+NUM_CORES:=$(shell cat /proc/cpuinfo | grep processor | wc -l)
 
 # Check for libraries and export cflags/linker variables
 NETTLE_FOUND    = $(shell if pkg-config --exists nettle; then echo "1" ; else echo "0"; fi)