• Improve performance when unserializing objects/arrays and serializing objects/arrays/strings in php 5/7.
• Update unserialization of integer object keys for php 7.2: Make those keys accessible when unserializing.
• Properly pick up presence of gcc for default compiler flags (cc --version doesn't contain gcc).
  Add -O2 to default gcc compiler flags.
• Use empty string for serializing empty $_SESSION array, similar to "session.serialize_handler=php".
  Older igbinary releases already unserialize the empty string to the empty array.