Hello Geoff!
About cmake:
1. You can still use the binary build for your system.
2. You can still build Postgres from source and with old gcc, you need only install cmake (it's very easy) Only most modern versions of CMake depend on modern gcc. I have good experience with old Solaris and AIX. (I mean build Postgres by current cmake branch).
3. You can try and put your impressions on issue tracker on github.
Thanks.