Because OGS building should work out of the box this should be set to `ON` per default. Also I suggest to put a the prefix `OGS_` in front of the variable. To be consistent with existing CMake variables (e.g. `OGS_NO_EXTERNAL_LIBS`) I would negate the meaning of the variable. To sum up:

    OPTION(OGS_NO_BOOST_DOWNLOAD "Always use the system installed Boost?" OFF)