Use the visibility attribute for newer GCC versions

In newer versions of GCC the exporting of symbols can be controlled with
an attribute if -fvisibility=hidden is used. This patch allows the
CIVETWEB_API macro to work with this flag.

include/civetweb.h

@@ -34,6 +34,8 @@
 #else
 #define CIVETWEB_API
 #endif
+#elif __GNUC__ >= 4
+#define CIVETWEB_API __attribute__((visibility ("default")))
 #else
 #define CIVETWEB_API
 #endif