auth: build with -fvisibility=hidden

[Habbie]

Short description

Checklist

I have:

• [ ] read the CONTRIBUTING.md document
• [ ] compiled this code
• [ ] tested this code
• [ ] included documentation (including possible behaviour changes)
• [ ] documented the code
• [ ] added or modified regression test(s)
• [ ] added or modified unit test(s)
• [ ] checked that this code was merged to master

[Habbie]

This appears to actually break ODBC.

[rgacogne]

I wonder if it's related to the "Problems with C++ exceptions" described in https://gcc.gnu.org/wiki/Visibility It might make hiding our symbols by default problematic in the authoritative server because it loads plugins at runtime.