tools iio: Override CFLAGS assignments

So user could specify outside CFLAGS values.

Cc: Jonathan Cameron <jic23@kernel.org>
Cc: Hartmut Knaack <knaack.h@gmx.de>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>