Fix opus_defines.h to define OPUS_EXPORT only when it's not defined.

include/opus_defines.h

 /* Copyright (c) 2010-2011 Xiph.Org Foundation, Skype Limited
    Written by Jean-Marc Valin and Koen Vos */
 /*
    Redistribution and use in source and binary forms, with or without
    modification, are permitted provided that the following conditions
    are met:
 
    - Redistributions of source code must retain the above copyright
    notice, this list of conditions and the following disclaimer.
 
@@ skipping 45 matching lines @@
 #define OPUS_UNIMPLEMENTED    -5
 /** An encoder or decoder structure is invalid or already freed @hideinitializer*/
 #define OPUS_INVALID_STATE    -6
 /** Memory allocation has failed @hideinitializer*/
 #define OPUS_ALLOC_FAIL       -7
 /**@}*/
 
 /** @cond OPUS_INTERNAL_DOC */
 /**Export control for opus functions */
 
+#if !defined(OPUS_EXPORT)
+
 #if defined(__GNUC__) && defined(OPUS_BUILD)
 # define OPUS_EXPORT __attribute__ ((visibility ("default")))
 #elif defined(WIN32) && !defined(__MINGW32__)
 # ifdef OPUS_BUILD
 #   define OPUS_EXPORT __declspec(dllexport)
 # else
 #   define OPUS_EXPORT
 # endif
 #else
 # define OPUS_EXPORT
 #endif
 
+#endif
+
 # if !defined(OPUS_GNUC_PREREQ)
 #  if defined(__GNUC__)&&defined(__GNUC_MINOR__)
 #   define OPUS_GNUC_PREREQ(_maj,_min) \
  ((__GNUC__<<16)+__GNUC_MINOR__>=((_maj)<<16)+(_min))
 #  else
 #   define OPUS_GNUC_PREREQ(_maj,_min) 0
 #  endif
 # endif
 
 #if (!defined(__STDC_VERSION__) || (__STDC_VERSION__ < 199901L) )
@@ skipping 537 matching lines @@
   * @returns Version string
   */
 OPUS_EXPORT const char *opus_get_version_string(void);
 /**@}*/
 
 #ifdef __cplusplus
 }
 #endif
 
 #endif /* OPUS_DEFINES_H */