tools gpio: Allow overriding CFLAGS

So that the user can specify outside CFLAGS values.

Signed-off-by: Jiri Olsa <[e-mail]>
Reviewed-by: Andy Shevchenko <[e-mail]>
Cc: Hartmut Knaack <[e-mail]> <[e-mail]>
Cc: Herton Krzesinski <[e-mail]>
Cc: Jonathan Cameron <[e-mail]>
Cc: Lars-Peter Clausen <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/gpio/Makefile

···
 # (this improves performance and avoids hard-to-debug behaviour);
 MAKEFLAGS += -r
 
-CFLAGS += -O2 -Wall -g -D_GNU_SOURCE -I$(OUTPUT)include
+override CFLAGS += -O2 -Wall -g -D_GNU_SOURCE -I$(OUTPUT)include
 
 ALL_TARGETS := lsgpio gpio-hammer gpio-event-mon
 ALL_PROGRAMS := $(patsubst %,$(OUTPUT)%,$(ALL_TARGETS))