Minor makefile change so that emerge can work.  This is for testing purposes only.

Review URL: http://codereview.chromium.org/839001
diff --git a/nvtool/Makefile b/nvtool/Makefile
index 10e504f..4403e8b 100644
--- a/nvtool/Makefile
+++ b/nvtool/Makefile
@@ -6,8 +6,8 @@
 
 OSNAME = $(shell uname)
 
-CC = gcc
-CFLAGS = -Wall -g -I. -L/usr/local/lib
+CC ?= gcc
+CFLAGS += -Wall -g -I. -L/usr/local/lib
 LIBS = -ltspi
 
 ifeq ($(OSNAME), Darwin)