Refactor for testing

Added basic decoding functionality to test.

Refactored some code to stop 3DS specific code from compiling on
GNU/Linux.

Tested working by running the test executable on GNU/Linux. Vorbis
decoding has many errors detected by valgrind (but still decodes
successfully).
Opus, flac, and mp3 decoding work without errors.

Signed-off-by: Mahyar Koshkouei <mk@deltabeard.com>

source/error.h

@@ -1,4 +1,6 @@
+#if defined __arm__
 #include <3ds.h>
+#endif
 
 /* Errors that can't be explained with errno */
 #define NDSP_INIT_FAIL			1000
@@ -6,6 +8,8 @@
 #define FILE_NOT_SUPPORTED		1002
 #define UNSUPPORTED_CHANNELS	1003
 
+#if defined __arm__
+
 /**
  * Struct to help error handling across threads.
  */
@@ -28,3 +32,5 @@ struct errInfo_t
  * \param err	Error number.
  */
 char* ctrmus_strerror(int err);
+
+#endif