used gcc for building osx; updated makefile accordingly

Makefile

@@ -1,4 +1,9 @@
-CC = gcc
+ifeq ($(shell uname -s),Darwin)
+	CC = /usr/local/Cellar/gcc/5.3.0/bin/gcc-5 
+else
+	CC = gcc
+endif 
+
 CFLAGS = -std=c99
 
 # DEBUGGING FLAGS