Possibility to build/link with non-system & non-vendored libbpf?

[danielocfb]

This came up internally. Folks were wondering if it's possible to build & link with a "custom" libbpf, anywhere on the file system. If not it may be useful to add that capability down the line.

[danielocfb]

cc @chantra

[danielocfb]

I think I now know the answer to this question, at least once #74 lands: if you build with static-libbpf but without vendored-libbpf the build will pick up the system's libbpf.a. You can set LD_LIBRARY_PATH (that variable name may need a revisit for static libraries...) to the directory that contains said file and everything should work as you'd expect.