Only expose public functions in GNU compiler builds if SM64_LIB_EXPORT is defined.

This is useful for WebAssembly builds, since `libsm64` is built as a static library and the final linked `.wasm` file might not want to expose the `libsm64` functions.

src/libsm64.h

@@ -12,10 +12,14 @@
         #define SM64_LIB_FN __declspec(dllimport)
     #endif
 #elif defined(__GNUC__) && __GNUC__ >= 4
+    #ifdef SM64_LIB_EXPORT
         #define SM64_LIB_FN __attribute__ ((visibility("default")))
     #else
         #define SM64_LIB_FN
     #endif
+#else
+    #define SM64_LIB_FN
+#endif
 
 #ifdef __cplusplus
 extern "C" {