3rd-party source code can't be built as shared library on Linux

[harokyang]

I added some source code of 3rd-party libraries into the project
Everything is OK if they are built with static link
But if build as shared library and dynamic linked on Linux platform, I got a lot of "undefined reference" error

After some investigation I found "-fvisibility=hidden" in root CMakeLists.txt, which turn off global symbol exportation on shared library by default
In Baikal source code we have: attribute((visibility ("default"))), so any specified symbol can still be exported

That's why any 3rd-party source code without attribute((visibility ("default"))) can't be built as shared library

And in my case, the worst part is:
This library did not define any API_ENTRY macro
On Windows, CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS is set, so CMAKE will help to export all symbols
On Linux, the default behavior of gcc is expected to export all symbols

So I have to modify all function declaration to make it work
Or just remove -fvisibility=hidden, and add it back as individual compiler flag to Baikal targets, which is more reasonable