Pull Request Overview

This PR refactors the dynamic linking code in the build script by creating a new FFmpegLinkMode enum to replace the inline string matching, improving type safety and code organization.

• Introduced a proper enum FFmpegLinkMode with From<String> and Display implementations
• Simplified the linking functions to use the enum's Display trait directly in cargo link commands
• Extracted common single library linking logic into a new linking_with_single_lib function

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