Move linking flags to build.rs to help with non-cargo build systems

[gferon]

While using rust_icu with Bazel, I realized that the #[cfg_attr] attributes in the code make everything difficult.

This brings the following improvement:

• Makes it easier to add combination of features to change the flags, since we can use proper branching
• Moving the instructions to the build.rs enables building rust_icu with Bazel (which can skip using the build.rs file)
• Adds the ability to specify the RUST_ICU_LINK_SEARCH_DIR environment variable when you need to use a non-installed build of icu4c
• Document the static feature and add a CI job to test this for both Ubuntu and macOS

[filmil]

Please rebase, and let me know when to continue reviewing.

[gferon]

This is ready for a review now!

[filmil]

Thank you for the contribution, for improving the linking recipes, and for the new workflow configuration!

All are most welcome.