Move compilation steps to copr/epel

[kyl191]

Interestingly, the ldap plugin is natively available for Fedora: https://src.fedoraproject.org/rpms/openvpn-auth-ldap

But the CentOS versions aren't updated, looks like dependencies are the issue.

Trying a copr version of it isn't too helpful: https://copr.fedorainfracloud.org/coprs/kyl191/openvpn-auth-ldap/
gcc-objc was removed in RHEL8, there's no EPEL replacement, so it has to be pulled in from an external repo. (copr supports this, thankfully)

re2c just isn't updated: https://src.fedoraproject.org/rpms/re2c
I can build it locally on CentOS 8, so that should be easier to solve.

[kyl191]

Add re2c to epel8: https://bugzilla.redhat.com/show_bug.cgi?id=1971366

Also requested to add the openvpn-ldap epel8, but that is difficult: https://bugzilla.redhat.com/show_bug.cgi?id=1971331

Once re2c is added, I can simplify the ldap build by just installing it, and actually start on updating the copr spec to work (eg the buildrequires might be out of date compared to what's in the compile ldap steps)