Downgrade gcc

[shindavid]

Note: this pull request is NOT complete. The LRUCache data structure needs to be specialized when using sharedptr values to use `std::atomic*instead ofoperator=` and similar.

[shindavid]

Or actually, maybe instead of touching LRUCache, I can create a wrapper around std::shared_ptr that has copy constructor and operator= overrides that dispatch to std::atomic_*().

[shindavid]

Downgrade no longer necessary, thanks to work by @xorvoid