Decouple some more from GLib.

Allows forward-declaration.

If we return false, their values are undefined.

Restore the "mpcdec" decoder plugin that was accidently broken since
commit 8ead8f7e.

Make the third argument to AC_CHECK_LIB() non-empty to disable its
default handler.

Replacing GLib macros.

Micro-optimization.

Merge duplicate symbol.