[libbpf] Add new port

[chris1786]

• Changes comply with the maintainer guide.
• The packaged project shows strong association with the chosen port name. Check this box if at least one of the following criteria is met:
  • The project is in Repology: https://repology.org/project/libbpf/versions
  • The project is amongst the first web search results for "" or " C++". Include a screenshot of the search engine results in the PR.
  • The port name follows the 'GitHubOrg-GitHubRepo' form or equivalent Owner-Project form.
• Optional dependencies of the build are all controlled by the port. A dependency is controlled if it is declared an unconditional dependency in vcpkg.json, or explicitly disabled through patches or build system arguments such as CMAKE_DISABLE_FIND_PACKAGE_Xxx or VCPKG_LOCK_FIND_PACKAGE
• The versioning scheme in vcpkg.json matches what upstream says.
• The license declaration in vcpkg.json matches what upstream says.
• The installed as the "copyright" file matches what upstream says.
• The source code of the component installed comes from an authoritative source.
• The generated "usage text" is brief and accurate. See adding-usage for context. Don't add a usage file if the automatically generated usage is correct.
• The version database is fixed by rerunning ./vcpkg x-add-version --all and committing the result.
• Exactly one version is added in each modified versions file.

[chris1786]

This library does not have a configuration step, and requires the build step to be executed within the source tree. I couldn't find a way to get vcpkg_make_configure or vcpkg_make_install to support this, so have implemented the required logic within the port itself.

[dg0yt]

This is easily solved by: Adding a configure script.
This script can capture configuration options and save them to a Makefile which is invoked for the actual build. Example: ports/libcap/configure. (ports/*/configure for more.)

[dg0yt]

There is even an Q&A: #49572