Enable user-overriable CFLAGS

Put required CFLAGS (ie pkg-config --cflags) in ALL_CFLAGS and append
user specified CFLAGS to that.

Use CFLAGS during link.

Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/libopenbmc_intf/Makefile b/libopenbmc_intf/Makefile
index 9cd7be6..8853318 100644
--- a/libopenbmc_intf/Makefile
+++ b/libopenbmc_intf/Makefile
@@ -2,11 +2,11 @@
 INSTALLDEPS=install-lib
 CLEANDEPS=clean-lib
 DEFAULT_ALL=$(LIBOBMC)
-CFLAGS+=-iquote ../gdbus
+ALL_CFLAGS+=-iquote ../gdbus
 LIBOBMC=openbmc_intf
 
 $(LIBOBMC): %: %.o gpio.o
-	$(CC) -shared $(LDFLAGS) -Wl,-soname,lib$(LIBOBMC).so \
+	$(CC) -shared $(CFLAGS) $(LDFLAGS) -Wl,-soname,lib$(LIBOBMC).so \
 		-o lib$@.so.1 $^ $(LDLIBS)
 
 install-lib: