Makefile: Do not override CC.

The makefile currently forces CC = gcc, which prevents building against
a Yocto SDK.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
diff --git a/Makefile b/Makefile
index ebf69f3..0755653 100644
--- a/Makefile
+++ b/Makefile
@@ -1,5 +1,5 @@
 
-CC = gcc
+CC ?= gcc
 CFLAGS = -Wall -Wextra -Werror
 INSTALL = install
 prefix = /usr/local