You'll need the mod_authnz_pam package that is available in RHEL 7.x. For a configuration example, see here: http://www.adelton.com/apache/mod_authnz_pam/

Warning: Since the password hashing methods available in typical http clients are not compatible with the password hashing scheme used in /etc/shadow, the module will need to receive the actual password from the client, not just a digest or a hash. So this must not be allowed over plain http, only with https. Otherwise, the client would be sending their unix account password unencrypted over the network, and that would make it really easy to collect the passwords by monitoring the network traffic.