This fixes an old bug which caused the "unused" warnings to be
unreliable; only the first block in the list was marked as being
"used", no matter if it was really used, and the rest was never marked
as "used", suppressing all warnings for them.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

The old struct config_param remains only for top-level string options.

libmp4v2 is licensed under MPL 1.1, which is incompatible with GPLv2.
Unfortunately, this means that we must remove the plugin.

More information can be found in the Debian bug report:

 http://bugs.debian.org/767504

Simplify the definition of many build options.

This plugin uses the MP4v2 library to play mp4/m4a files.
It is limited to file_decode.

Merge duplicate code.

Replaces decoder_plugin_from_suffix().