More Solaris/OpenIndiana fixes. All backends compile now.

* libtool this old doesn't probe sed for us.
* Do not redefine _GNU_SOURCE in the AC_CHECK_TYPES probe.
* devpoll and evport need signal.c.  This means *all* current backends need
  signal.c, but I'm keeping the opt-out around for now.
Merge branch 'include-hygiene' of github.com:zackw/Libevent into include-hygiene