I'd second Adam's point. Any resip cycles we might have should be spent on what Adam has outlined below first.
We should have some form of help text, that enables someone to migrate to the new build system as painlessly as possible. In order to do this, people must understand how to do the things they used to do with the old ./configure script. This includes:
1. An example of how to switch tool chains / cross compile.
2. An example of how to switch from debug builds to opt builds.
3. How to build shared libraries vs show to build static libraries.
4. How to use DISTCC
5. How to use CCACHE
6. How to build MySQL support into repro
7. How to build dum with radius client support
8. How to build tfm
9. How to build recon
10. How to build reTurn server
11. How to build reTurn client
12. How to build with or without OpenSSL
13. How to build with or without DTLS support
14. How to build with CURL
15. How to build with Google Malloc
16. How to build with Google CPUPerf
17. How to build with Debug Malloc
18. How to build with Electric Fence
19. How to build with Random Thread Local option enabled
20. How to build with IPv6 enabled
21. How to build with EPoll support
22. How to build with POPT support
23. How to build with Fixed Point only option enabled
24. How to build with Pedantic Stack option
25. How to build with Dum Thread Debug on
26. How to build with CARES
All these options are from the old configure script. I know you've covered some of these with your examples, but it would be great to cover them all in some fashion.