cmake: Make the WITH_DOC (Doxygen) optional, defaulting off
cmake: Make the WITH_DOC (Doxygen) optional, defaulting off

This saves nominal build time by a second.
To turn on Doxygen, just pass the -DWITH_DOC flag to cmake.
Now will error if it can not find Doxygen.

Signed-off-by: Robin Getz <c66e5a86ef9317cf35cecb2ed063996b2100ed4b@analog.com>

analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio