[Linux] Only export extension init symbol

[Faless]

Since we link with static libstdc++ we need to tell gcc to only export the necessary symbols.
Using "-fvisibility=hidden" will not work, since libstdc++ explicitly export its symbols.

Fixes #127 .

[Sch1nken]

I don't know how flexible Sconstruct is, but could the build system potentially read the .gdextension file to determine the entry_symbol and generate the symbols.map file during compilation?

This is not really related to this WebRTC Extension anymore, but I wonder if this is a good "default" behaviour for new extension (i.e. the demo repository from Godot for GDExtensions). Or alternatively provide a flag that enables/disables the version script usage.

[Faless]

I don't know how flexible Sconstruct is, but could the build system potentially read the .gdextension file to determine the entry_symbol and generate the symbols.map file during compilation?

It's possible, I've been thinking about creating a GDExtension tool for SCons for a while, which should ideally also generate the .gdextension file.