Except for arginfo header as PHP_FUNCTION/PHP_METHOD are marked as U_CFUNC and the `ext/date/php_date.h` header doesn't use extern C.
We now specificy the header symbols that _need_ to follow the C function ABI, meaning in some places we drop U_CFUNC declarations and in others we add it.
Also use the `BEGIN_EXTERN_C()` portability macro when possible so that searching for this is easier.
There are several issues with this macro:
- It is incorrectly named: It's not an empty default case. It's an unreachable
default case.
- It is hiding control flow in its definition, which can be confusing for
humans and tools (such as Coccinelle) alike, because it looks like it would
be a statement that belongs to the "current" case rather than opening a new
one.
Since this macro is exactly as long as explicitly spelling out its definition
(excluding the useless `break;`), it is not even making the code any more
succinct.