BUILD: enable poll() by default in the makefile

This allows to build haproxy for unknown targets and still have poll().
If for any reason a target does not support it, just passing USE_POLL=""
disables it.
BUILD: add explicit support for Mac OS/X

The "osx" target may now be passed in the TARGET variable. It supports
the same features as FreeBSD and allows its users to use the GNU makefile
instead of the platform-specific makefile which lacks some features.