CMake: Allow to override target lib dir
CMake: Allow to override target lib dir

This is useful for creating Debian packages, as Debian stores the
libraries in arch-specific folders.

Signed-off-by: Paul Cercueil <73d292422978d45c9eeef455bb1b24a0db453722@analog.com>

analogdevicesinc/libad9361-iio,analogdevicesinc/libad9361-iio,analogdevicesinc/libad9361-iio,analogdevicesinc/libad9361-iio