The configure order for apr does netinet/sctp.h before sys/socket.h. Solaris requires sys/socket.h be included for sctp.h to be properly detected. Re-arranging it so that socket.h is detected before sctp.h should solve the problem and allow for Apache to utilize SCTP on Solaris properly.
*** This bug has been marked as a duplicate of 35021 ***
The referenced 'duplicate' did fix the code. It did not fix autoconf. Currently we aren't detecting sctp.h because the autoconf-foo doesn't know to include sys/socket.h in the process of testing for sctp.h; sctp.h won't compile. There's a pretty big red warning on solaris 10, at least, in ./configure.
Actually, while this is kind of ugly, it turns out to not be an actual problem. The presence of netinet/sctp.h is detected, and that's what's used later on when determining if we should use it, and at that point we do include sys/socket.h first, so it does work. It would certainly be nice if we didn't get that scary warning, but it's not a bug in the sense that the SCTP support does still end up turned on, at least as far as I can tell.