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.
Gina Peter Banyard