Support static linking with FMOD

Closes #241

panda/src/audiotraits/config_fmodAudio.cxx

@@ -51,6 +51,8 @@ init_libFmodAudio() {
   FmodAudioManager::init_type();
   FmodAudioSound::init_type();
 
+  AudioManager::register_AudioManager_creator(&Create_FmodAudioManager);
+
   PandaSystem *ps = PandaSystem::get_global_ptr();
   ps->add_system("FMOD");
   ps->add_system("audio");

panda/src/audiotraits/config_fmodAudio.h

@@ -24,7 +24,7 @@ NotifyCategoryDecl(fmodAudio, EXPCL_FMOD_AUDIO, EXPTP_FMOD_AUDIO);
 
 extern ConfigVariableInt fmod_audio_preload_threshold;
 
-extern EXPCL_FMOD_AUDIO void init_libFmodAudio();
+extern "C" EXPCL_FMOD_AUDIO void init_libFmodAudio();
 extern "C" EXPCL_FMOD_AUDIO Create_AudioManager_proc *get_audio_manager_func_fmod_audio();
 
 #endif // CONFIG_FMODAUDIO_H