fix compile warnings
[descriptor_builder] split large block into individual function
[descriptor_builder] refactor functions
[descriptor_builder] refactor php_protocolbuffers_build_fields
[protocolbuffers] no longer support array base descriptor.

Actually, Protocolbuffers supported array based descriptor.
But keeping that codes is trobulesome. so I decided to drop that feature.

class Person
{
    public function getDescriptor()
    {
        // please use descriptor builder and descriptor class.
        return array(
            "tag" => 1,
            "name" => "user_id",
            //...
        );
    }
}

this change doesnt affect most people. maybe only for me.
[config.m4] comment out CFLAGS