Updated GNU make Makefile for cross compile by changing CC=gcc to CC:=gcc.

bacnet-stack/lib/Makefile

@@ -1,10 +1,10 @@
 #Makefile to build BACnet Library with GCC
 
 # tools 
-#     ('?=' sets them to gcc and ar unless they are already defined for
+#     (':=' sets them to gcc and ar unless they are already defined for
 #           a board-specific cross-compiler and archiver)
-CC ?= gcc
-AR ?= ar
+CC := gcc
+AR := ar
 
 # target
 TARGET = bacnet