Remove Mac poll from cmake build #1089

Ref #1057

poll doesn't work right on some Mac versions so it's been disabled for all Mac in the autotools build. My understanding similar changes are needed for the cmake build. It was suggested I start a separate issue for that, so here's a c&p of my question in that thread:

for example should it be

if(NOT APPLE)
check_symbol_exists(poll          "${CURL_INCLUDES}" HAVE_POLL)
endif()

also should there be some sort of failsafe in setup like

#if defined(__APPLE__) && (defined(HAVE_POLL) || defined(HAVE_POLL_FINE))
#error "failsafe: we don't allow poll on apple, build config should have detected this"
#endif

this way it doesn't get away from us as more build systems are added or say for example someone adds HAVE_POLL_FINE to cmake

basically the idea is we want to make sure no poll on mac

@jzakrzewski @snikulov @Lekensteyn