On non-Windows systems, allow `import code from library "-l..."` to link shared libraries

Merged Camil Staps requested to merge import-code-from-library-for-shared-libraries into master

This is useful when you combine libraries that both rely on different shared libraries. Previously, you would have to create a new environment with linker flags for both. With this solution, the libraries can use import code from library "-l..." to add what they need and this is combined automatically without the need for a new environment.

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