fix the link step on Windows

RtAudio.h

@@ -48,7 +48,11 @@
 #define RTAUDIO_VERSION "5.0.0"
 
 #if defined _WIN32 || defined __CYGWIN__
-  #define RTAUDIO_DLL_PUBLIC
+  #if defined(RTAUDIO_EXPORT)
+    #define RTAUDIO_DLL_PUBLIC __declspec(dllexport)
+  #else
+    #define RTAUDIO_DLL_PUBLIC
+  #endif
 #else
   #if __GNUC__ >= 4
     #define RTAUDIO_DLL_PUBLIC __attribute__( (visibility( "default" )) )

rtaudio_c.h

@@ -2,7 +2,7 @@
 #define RTAUDIO_C_H
 
 #if defined(RTAUDIO_EXPORT)
-#ifdef WIN32
+#if defined _WIN32 || defined __CYGWIN__
 #define RTAUDIOAPI __declspec(dllexport)
 #else
 #define RTAUDIOAPI __attribute__((visibility("default")))