I think this issue may have gotten some attention as I no longer see a build error while compiling PHP, but it does not install memcached properly. This issue also affects PHP-FPM7.3 as well. In fact, this is where I noticed the problem as PHP-FPM 7.3 does not have a working memcached extension when it is selected in the build options. It used to throw several errors while compiling; now it does not seem to be doing that, yet still doesn't work.
Could someone take a look at this and finish a fix for it in both PHP7.3 and PHP-FPM7.3.