Add example of linking against a C++ shared library

[domfarolino]

fixes #294 and contributes to #328.

Please take a look @budziq, I'd love for the cookbook to have a simple example of Rust dynamically linking against a C/C++ shared library through a C ABI.

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Things to check before submitting a PR

• the tests are passing locally with cargo xtask test all
  • Note: almost every test fails for me locally. I'm not sure why, but I'm pretty sure it is not related to my commits.
• commits are squashed into one and rebased to latest master
• PR contains correct "fixes #ISSUE_ID" clause to autoclose the issue on PR merge
• non rendered items are in sorted order (links, reference, identifiers, Cargo.toml)
  • Question: Sorted how? Lexicographically, or by use (i.e., link reference/use)
• links to docs.rs have wildcard version https://docs.rs/tar/*/tar/struct.Entry.html
• example has standard error handling
  • My liberal use of .unwrap() in basic helper code I think is OK here? But I'd love for someone to confirm!
• code identifiers in description are in hyperlinked backticks

[`Entry::unpack`]: https://docs.rs/tar/*/tar/struct.Entry.html#method.unpack

[AndyGauge]

Thanks for the contribution! This is a great addition to the cookbook, but there are several technical and naming inconsistencies that need to be addressed before it's ready for merge:

1. Title vs. Content: The section header is titled "Compile and link statically...", but the content describes dynamic linking to a shared library (.so).
2. Filename Inconsistencies:
   • The prose mentions src/mylibrary.h and src/mylibrary.cc.
   • Code blocks are labeled src/library.c and src/library.h.
   • The Makefile expects src/mylibrary.c and src/mylibrary.h.
   • src/library.c includes "api.h", which isn't defined (should likely be library.h or mylibrary.h).
3. Language/Compiler Mismatch: The implementation file is labeled .c but contains C++ code (<iostream>, std::cout). It should probably be named .cpp or .cc, and the Makefile should reflect this.
4. Build Search Path: cargo:rustc-link-search=native=. assumes the library is in the root during build. While simple, it's often better to reference CARGO_MANIFEST_DIR or provide instructions on where the user should run the Makefile to ensure the .so is found.

Requesting changes to align the filenames, headers, and terminology!