Compile in C++11 mode.
Declare some plugin headers without named initializers.
adplug: Convert C parts to C++.
aac: Compile as C++.
aac-raw: Compile as C++.
alsa: Compile as C++ (and some cleanups).
amidi-plug: Compile as C++.
Compile playlist plugins as C++.
cdaudio-ng: Compile as C++.
console: Compile as C++.
ffaudio: Compile as C++.
filewriter: Compile as C++.
flacng: Compile as C++.
gio: Compile as C++.
jack: Compile as C++.
metronom: Compile as C++ (no changes needed).
mms: Compile as C++.
modplug: Compile as C++.
mpg123: Compile as C++.
neon: Compile as C++.