Fix 'openssl-skip-update' make target not found with older CMake versions
Fix 'openssl-skip-update' make target not found with older CMake versions

There's no real need to specify UPDATE_COMMAND or UPDATE_DISCONNECTED since as far as I can tell there is no default update step for URL sources, so there's no update that we need to skip.

In older CMake versions, specifying a custom update step with/without UPDATE_DISCONNECTED could cause this error.

CMake bug: https://cmake.org/Bug/view.php?id=15904

Closes #208

zhaozg/luvi,luvit/luvi,luvit/luvi,zhaozg/luvi