This change adds two configuration options:
--with-eventloop=[glib|internal|auto]
--with-pollmethod=[epoll|auto]
First allows switching between GLib event loop and internal one.
Second chooses backend to use for internal event loop.
Conditional compilation symbols are changed accordingly.
Additional helper macro MPD_OPTIONAL_FUNC_NODEF is added as well.
Remove the runtime check for eventfd(), hard-code the feature once
it's been selected at compile time. The class WakeFD is splitted into
EventFD and EventPipe, using WakeFD as a macro diversion.