Switch from C89 compatibility to C99, and comply to it

As revelated by the -pedantic flag, libiio isn't quite compatible
with C89 for several reasons, some of which would require big changes.
For instance, C89 does not support the type "long long".

Libiio is now compiled for C99 by default, and fully complies to
the standard.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
Simplify prototype of function iio_buffer_step

The iio_channel argument was never used.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>