Add novendor build flag to use system installed libbpf

[danobi]

This commit adds support to build libbpf-sys using the system installed
libbpf instead of building and linking against the vendored sources.

This is necessary to get libbpf-sys packaged into (most) distro's
package repositories. For example, Fedora has this requirement:
https://fedoraproject.org/wiki/Bundled_Libraries

This closes #10 .

[danobi]

Test plan

With this docker container:

FROM fedora:34

RUN dnf update
RUN dnf install -y curl gcc pkgconf-pkg-config
RUN curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
RUN dnf install -y libbpf-devel

ENTRYPOINT ["/bin/bash"]

Running:

$ podman build -t fedora -f Dockerfile
$ podman run -v $(pwd):/libbpf-sys -it fedora
[root@6f479fd4ee9a /]# cd libbpf-sys
[root@6f479fd4ee9a libbpf-sys]# cargo test --features novendor
<...>
running 1 test
test tests::test ... ok
<...>
[root@6f479fd4ee9a libbpf-sys]# ldd ./target/debug/deps/tests-f9beebfff4efee33
<...>
        libbpf.so.0 => /lib64/libbpf.so.0 (0x00007fed68ef6000)
<...>
        libelf.so.1 => /lib64/libelf.so.1 (0x00007fed68b83000)
        libz.so.1 => /lib64/libz.so.1 (0x00007fed68b69000)

[alexforster]

Concerned this will get out of sync with the crate version – I'll certainly forget to bump it! Can we instead do something like–

    .atleast_version(&format!("{}.{}.{}", env!("CARGO_PKG_VERSION_MAJOR"), env!("CARGO_PKG_VERSION_MINOR"), env!("CARGO_PKG_VERSION_PATCH")))

Since I guarantee that the major/minor/rev numbers will match the libbpf version exactly, I think this should Just Work™.

[danobi]

Updated

[alexforster]

Looks good. Once Travis runs I'll merge and cut a release.