Added line to makefile that aborts when BINARY_C is not set

Makefile

@@ -2,6 +2,10 @@
 
 # you will need to set the BINARY_C variable to point
 # to the root binary_c directory
+ifeq ($(BINARY_C),)
+  $(error BINARY_C is not set)
+endif
+
 
 CC      := gcc
 LD      := gcc