WITH_PMEM: Change the parameter from BOOL to a ternary STRING
with a default value that allows to use libpmem when it is available.
In this way, a libpmem dependency will be automatically installed
on other than Debian-based systems, based on what is available on
the continuous integraton system for building the packages.

WITH_PMEM=auto (default): Use libpmem when available.
WITH_PMEM=yes: Require libpmem to be available.
WITH_PMEM=no: Do not depend on libpmem.