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.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>