Fix build using the CMake Ninja backend
Fix build using the CMake Ninja backend

The Ninja build system needs to know about the outputs of the external
projects that are built, otherwise it fails with this sort of error:

    ninja: error: 'ncurses-prefix/src/ncurses/lib/libncurses.a', needed by
    'userspace/sysdig/csysdig', missing and no known rule to make it

The BUILD_BYPRODUCTS argument allows us to create the correct dependency
rules.

sysdig-CLA-1.0-signed-off-by: Sam Thursfield ssssam@gmail.com

angeloc/sysdig,angeloc/sysdig,angeloc/sysdig,angeloc/sysdig,angeloc/sysdig