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

include/opus_defines.h

Index: include/opus_defines.h
diff --git a/include/opus_defines.h b/include/opus_defines.h
index d86fbccf0fddcfc5b314c56f9c12990dbc8f93ef..ec154ba15a05bec7c6ce1c7e12bf78abae33c1bb 100644
--- a/include/opus_defines.h
+++ b/include/opus_defines.h
@@ -63,6 +63,8 @@ extern "C" {
 /** @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__)
@@ -75,6 +77,8 @@ extern "C" {
 # define OPUS_EXPORT
 #endif
 
+#endif
+
 # if !defined(OPUS_GNUC_PREREQ)
 #  if defined(__GNUC__)&&defined(__GNUC_MINOR__)
 #   define OPUS_GNUC_PREREQ(_maj,_min) \