Allow CFLAGS to be extended from the environment

We also remove -g, since we don't necessarily want to build with
debugging symbols. Buildroot will add -g if needed.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Index: b/Makefile
===================================================================
--- a/Makefile
+++ b/Makefile
@@ -9,7 +9,7 @@
 
 include version.mk
 
-CFLAGS = -g -Wall $(IPR_DEFINES)
+override CFLAGS += -Wall $(IPR_DEFINES)
 UTILS_VER = $(IPR_MAJOR_RELEASE).$(IPR_MINOR_RELEASE).$(IPR_FIX_LEVEL)
 TAR = cd .. && tar -zcpf iprutils-$(UTILS_VER)-src.tgz --exclude CVS --exclude applied-patches --exclude series --exclude txt --exclude pc --exclude patches --exclude debug --exclude *~* iprutils