Fix GH-9518: Disabling IPv6 support disables unrelated constants

We need to properly terminate the "conditional compilation" in stub
files.

Closes GH-9520.

Add parenthesis around preprocessor conditions in stubs

gen_stub.php concatenates nested #ifs into one #if so let's make sure the semantics remain the same.

Declare ext/standard constants in stubs - part 2 (#9426)