libiio: Bump version to 0.7

libiio: Bump version to 0.7

The two previous patches have been merged upstream, so they have been
deleted here. Another patch (picked from upstream) has been added to fix
the build with thread-less toolchains.

Libiio v0.7 provides two new backends, a USB backend (using libusb-1.0)
and a serial backend (using libserialport).

Additionally, it is now possible to compile libiio with thread-less
toolchains. In that case, thread safety is disabled.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>