Do not export internal symbols from a unix shared library.

include/GL/glfw3.h

@@ -157,6 +157,10 @@ extern "C" {
   #define GLFWAPI __declspec(dllimport)
  #endif
 
+#elif defined(__GNUC__) && defined(_GLFW_BUILD_DLL)
+
+ #define GLFWAPI __attribute__((visibility("default")))
+
 #else
 
  /* We are either building/calling a static lib or we are non-win32 */