Doing a runtime check in configure.ac (AC_RUN_IFELSE) has several
disadvantages:

* It doesn't work when cross-compiling. For example, if we build macOS
  binaries on a Linux system, we'd assume that poll() works, but in
  fact it won't.

* It checks the build system capabilities, but that is not necessarily
  appropriate if (for example) a macOS 10.10 user builds binaries that
  could be used by macOS 10.12 or macOS 10.9 users.

* It checks for one specific failure mode, but macOS seems to have a
  history of various implementation issues in poll().

* If we want it to work in CMake, we have to duplicate it in the CMake
  build system.

None of these is a showstopper on its own, but the combination of all
of them makes the current approach to avoiding the broken poll() on
macOS look unreliable. libcurl, a widely-portable library making
extensive use of sockets, specifically doesn't use poll() on Darwin
(macOS, iOS, etc.) or on Interix; let's follow their example here.

See also https://bugzilla.gnome.org/show_bug.cgi?id=302672 and
https://daniel.haxx.se/blog/2016/10/11/poll-on-mac-10-12-is-broken/


for some relevant history.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Resolves: #232