libiio: Drop requirement of threading support in the toolchain

The libiio library will compile fine with a toolchain that doesn't
support threads (tested with br-arm-full-nothread.config).
Only the IIOD program requires support for threading.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>