Added support for GCC 7 and GCC 8 and upgraded xapian-core.

Added

• GCC 7 and GCC 8 support (and libstdc++)

Changed

• Refactored pfh module
• Using ENUM() with minimal perfect hash functions (improves performance)
• Upgraded xapian-core to latest 1.5 (git@84ccecdfb8938b4011aac0bf9539c139204acd22)