Previously this was done with the generic autotools support, but CMake doesn't
have a standard option so set it explicitly.
(From OE-Core rev: 47bd456c1be169fc86788edf0173847b9b0d255b)
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>