For those of you in the future who may struggle with building Xymon on RHEL 8..

rrdtool-devel is not in the main repositories for RHEL 8. To install you need to enable the "CodeReady Builder for RHEL 8" repository.

dnf config-manager --set-enabled codeready-builder-for-rhel-8-x86_64-rpms

After doing this, a "dnf install rrdtool-devel" should be successful. Hope this saves someone some time.

Erik