Configure: I remove adding of -D_XPG4_2 -D__EXTENSIONS__ in sctp builds for

following reasons:

- it's not the way to engage XPG4v2 mode, defining _XOPEN_SOURCE to
  value less than 500 is (see standards(5));
- we need to work out strategy to handle _XOPEN_SOURCE, current state
  when we define e.g. _XOPEN_SOURCE to 500 in some files is inappropriate;
- sctp implementation on Solaris is incomplete, in sense that bss_dgram.c
  doesn't compile, because not all structures are defined, so that
  enabling sctp doesn't work anyway;