The build process in general should not be downloading at make time.
Specifically I hit a problem when my Ubuntu 18.04 did not have libmemcached installed and I tried building the platform from clean.
I hit problems at make time (not cmake time) like this:
The make process had downloaded a version of libmemcached that hit compile issues.
We shouldn't automatically download sources like this.
If it's an option to download sources, I think it should be at configuration time (e.g. cmake) not make time.
MEMCACHED_USE_EXTERNAL_LIBRARY should be default to OFF