Oracle is written in two layers, there is an abstraction layer and Oracle itself. If you know Oracle then you know it handles a lot of stuff for itself that the OS would traditionally do, for example it has its own process scheduling, interprocess communication, filesystem/volume manager, memory management - as of 11g it even talks directly to NFS servers in userland, bypassing the OS filesystem mounts entirely. Once the abstraction layer is ported then yes, it would be just "make" for Oracle itself.

This is how back in the day Oracle ran on 90+ Unix variants...

That was Informix, not Oracle.

Because of different shared memory optimizations in Solaris (especially Solaris 10+) and Linux, there's a non-trivial amount of work involved. I doubt it was anything close to "just type make".

I thought Oracle used Jam, not Make.