It looks like the spec file is generated automatically by the makepanda build script. In the top portion of MakeInstallerLinux is the line
WriteFile("panda3d.spec", txt)and slightly higher up are a couple of here-document definitions for INSTALLER_SPEC_FILE.
I admit I’ve not tried it myself, but running makepanda with --installer “should” trigger generation of the .spec file as well as the rest of the RPM build process.
–jonah