iio: make tools more cross-compilation friendly

When cross-compiling the IIO tools we need to opportunity to
specify a cross compiler prefix and some extra CFLAGS. This
patch enables this in the same way as for other stuff in
tools.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>