libiio: remove iio_context_valid usage

It's been removed from the master branch for upstream libiio. A null
pointer is returned when the network context cannot be created so we
check for that instead.

Signed-off-by: Tim Harder <timothy.harder@analog.com>
Filter Wizard: Use push buttons instead of toggle buttons

We don't handle making sure the toggle button is in a sane state in
relation to errors or what's happening in the GUI (i.e. the button
should never be in a depressed state when a user clicks on it to do
something like connect to target) so we'll just use simpler push buttons
for now specifically for connect to target, read clock settings, and
coefficients to target buttons.

Signed-off-by: Tim Harder <timothy.harder@analog.com>
libiio: update iio.h to match libiio master branch

Signed-off-by: Tim Harder <timothy.harder@analog.com>
libiio: use new iio_library_get_version fcn to get local lib version

Using iio_context_get_version with a null context pointer causes
segfaults at least against upstream libiio master.

Signed-off-by: Tim Harder <timothy.harder@analog.com>
libiio: remove __attribute__ usage from iio.h

Matlab's gcc compiling setup doesn't appear to work with this properly.
There might be a better way to specify certain compiler flags or
something different in a local mexopts.sh file to alter compile options.

Signed-off-by: Tim Harder <timothy.harder@analog.com>
libiio: bump minor version for local lib check to match upstream libiio

Signed-off-by: Tim Harder <timothy.harder@analog.com>