Fix definition of TL_API under GCC

[sebastinas]

No description provided.

[njh]

@sebastinas thank you very much for this fix.

Do you know what I could have done to pick this up before releasing?
Travis is configured to build against GCC 5.4.0 on Ubuntu:
https://travis-ci.org/njh/twolame/jobs/598281337

And I have my own build job here:
https://ci.aelius.com/job/twolame/95/console

Which runs on Debian 10, with GCC 8.3.0

Is there an extra test that I need to do?

[sebastinas]

What I did to find this issue is to check whether the .so only exports the functions defined in the public header (you can do that with nm -D and checking for the symbols marked with T). First, I noticed that too many symbols are exported. Then I've built with -fvisibility=hidden and ended up with no symbols. This uncovered the issue in the definition of TL_API. With that fixed, I noticed that one function was still missing.

[lazka]

We also needed this patch for the Windows build: msys2/MINGW-packages#5919