This replaces the old `dead` flag which was unreliable; it was `false`
if the EventThread was not yet started, which could cause deadlocks in
BlockingCall().
Since we switched from autotools to Meson in commit
94592c1406, we don't need to include
`config.h` early to properly enable large file support. Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.
This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.
Change EventLoop::IsInside() call to EventLoop::IsInsideOrNull().
This means that BlockingCall() may be used during shutdown, after the
main EventLoop::Run() has finished. This is important because mixers
are currently registered in the main EventLoop.
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.
