Low: build: introduce "--without esmtp" rpmbuild option

libesmtp-devel is not available in RHEL or CentOS, but only in
EPEL. For those who do not wish to install EPEL packages, introduce a
"--without esmtp" build option so the RPM build does not fail with an
unresolvable build dependency.

Mercurial revision: 936a367e828aa3535fd68edcdd3a59dd23719aef