Export C API symbols when compiled with g++

Makefile.am

@@ -6,6 +6,7 @@ endif
 AM_CXXFLAGS = @visibility@
 
 lib_LTLIBRARIES = %D%/librtaudio.la
+%C%_librtaudio_la_CXXFLAGS = -DRTAUDIO_EXPORT
 %C%_librtaudio_la_LDFLAGS = -no-undefined -export-dynamic -version-info @SO_VERSION@
 %C%_librtaudio_la_SOURCES = \
   %D%/RtAudio.cpp \

rtaudio_c.h

@@ -2,8 +2,12 @@
 #define RTAUDIO_C_H
 
 #if defined(RTAUDIO_EXPORT)
+#ifdef WIN32
 #define RTAUDIOAPI __declspec(dllexport)
 #else
+#define RTAUDIOAPI __attribute__((visibility("default")))
+#endif
+#else
 #define RTAUDIOAPI //__declspec(dllimport)
 #endif