Allow systemlib function declarations to use reserved words for function names

Summary: This allows builtin classes to define methods like "include" or "declare".
Necessary for replicating PHP5 extension interfaces in HNI.

  class Foo {
    public function include(string $file): bool {
      include($file);
      return true;
    }
  }

  (new Foo)->include("/bar/baz.php");

Closes #2022

Reviewed By: @paulbiss

Differential Revision: D1658153

Signature: t1:1658153:1415127992:e16e66ff2b5f6bef37e56be7e58d9a0fe86af4d3
Remove uspoof SINGLE_SCRIPT test.

Summary: It's deprecated on ICU >= 51

Closes #3925

Reviewed By: @JoelMarcey

Differential Revision: D1658533

Signature: t1:1658533:1415134838:c7bd38f7fdfed37acbcee497da7e7aed94d16850
Explicitly set -Wno-unused-function in gcc flags

Summary: This should be the default, but for systems
where it isn't, explicitly say so.

Closes #2515

Reviewed By: @JoelMarcey

Differential Revision: D1658664

Signature: t1:1658664:1415134757:d4d26bec54469d0bee379d4134c4d3c3e98b3dd1