Drop Apple-specific detection logic for `poll()`. This detection snippet
has been disabled for Apple in both configure and cmake, for `poll()`
being broken on Apple since 10.12 Sierra (2016).
Also replace `exit(1);` with `return 1;` in configure, to make the
snippets match.
Added in 9297ca49f5#1057 (2016-10-11).
Disabled for:
configure/darwin in a34c7ce754 (2016-10-18)
cmake/macOS in 825911be58#7619
cmake/iOS in d14831233d#8244
cmake/all Apple in a86254b393#12515Closes#14718
cc8b813765