1) __APPLE_USE_RFC_3542 should have been in socketmodule.c from the start, not sure why it was added in setup.py.

2) as you and Ned noticed -search_paths_first has been the default for a long while, we can just drop it and anyone building on ancient systems can add the flag to CFLAGs/LDFLAGS as needed.  IIRC we added the flag to be able to build with local copies of libraries also shipped in the OS, in particular when using static libraries for those local copies.

3) I don't know about detect_dbm_gdbm, and don't use those libraries myself. It would be nice to be able to open system files created using dbopen etc, but on the other hand there is at least one bug report complaining about data corruption when using one o the dbm extensions linked with the system version of the library.

4/5) Fine by me, although I'm slightly worried about using pkg-config because the system doesn't ship that tool. 

Something you don't mention is the logic dealing with SDK roots. I haven't checked yet if similar logic would be necessary in configure. With some luck it isn't, but that depends on what's supported by autoconf and the particular probes we want to use.

@Ned: Not being able to use system versions of libraries is a bit annoying, but that's something you already have to do due to openssl.  Maybe we should also try to clean up and refactor the build-installer.py script to do have a way to build/install those 3th-party dependencies.