Samba 4.9.0 ./configure lmdb error

The actual dependency to install (for Red Hat Enterprise Linux 7 / CentOS 7 / Scientific Linux 7) is lmdb-devel.

Rather than following some random tutorial for a now EOL version of Samba, you might be better off following the official (and up to date) Samba guidance: Build Samba from Source, Package Dependencies Required to Build Samba, Setting up Samba as an Active Directory Domain Controller

To be sure, the tutorial to which you linked DOES NOT list the current Samba dependencies for CentOS 7.

Everyone is giving you stick for trying to build from source however this is the only way if you want to run SambaAD as specified in all the offical samba guides people have linked.

"Red Hat Enterprise Linux 7 does not include all required packages to build a Samba AD DC. Enable the external Extra Packages for Enterprise Linux (EPEL) repository before you install the packages. For details, see https://fedoraproject.org/wiki/EPEL. Enabling the EPEL repository is not required on CentOS 7 and Scientific Linux 7."

@user2827958 make sure you've got the EPEL repository before you try and install all the packages