Since we switched from autotools to Meson in commit
94592c14, 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.

Only _WIN32 is defined by the compiler, and WIN32 is not standardized
and may be missing.

Closes #169

Fixes hanging child process in the "pipe" output plugin.

Users now have to call Close() explicitly.  This simplifies using the
class, as most users have automatic socket management already, and
Steal() had to be used often.

Allows building without GLib.  This fails to compile currently,
because GLib is still used in the MPD core.

Eliminate the global variable "shutdown_loop".