Make HL_API/HL_PRIM function have default visibility.
# define EXPORT __declspec( dllexport )
# define IMPORT __declspec( dllimport )
#else
+#if defined(HL_GCC) || defined(HL_CLANG)
+# define EXPORT __attribute__((visibility("default")))
+#else
# define EXPORT
+#endif
# define IMPORT extern
#endif
