[build] Fix building on gcc 3

GCC did not support #pragma GCC visibility until version 4.0.

src/include/compiler.h

  * This is a stronger claim than specifying "-fvisibility=hidden",
  * since it also affects symbols marked with "extern".
  */
+#if __GNUC__ >= 4
 #pragma GCC visibility push(hidden)
+#endif
 
 /** @def DBG
  *