Makefile: allow $(CFLAGS), $(LDFLAGS) override

The caller might have specified CFLAGS or LDFLAGS. Let's respect those.
Additionally add prefix var declaration defaulting to /usr/local.

Author: JamiKettunen

Makefile

@@ -1,7 +1,8 @@
 OUT := qmi-ping
 
-CFLAGS := -Wall -g -O2
-LDFLAGS := -lqrtr
+CFLAGS += -Wall -g -O2
+LDFLAGS += -lqrtr
+prefix = /usr/local
 
 SRCS := qmi_ping.c qmi_test.c