tools gpio: Allow overriding CFLAGS

So that the user can specify outside CFLAGS values.
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Hartmut Knaack <knaack.h@gmx.de> <knaack.h@gmx.de>
Cc: Herton Krzesinski <herton@redhat.com>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Link: http://lkml.kernel.org/r/20181212102537.25902-7-jolsa@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/gpio/Makefile

@@ -12,7 +12,7 @@ endif
 # (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))