Make symbols visible by default.

Default xcode build settings for Release builds cause symbols to become
private, preventing extensions from calling functions (e.g. vital Lua
functions) from Mjolnir. We now invert the GCC_SYMBOLS_PRIVATE_EXTERN
build setting to "NO" to allow this to work.

Fixes: #397
Merge pull request #402 from cmsj/fix-397

Fixes #397