Add a LIBGIT2_NO_VENDOR environment variable to build.rs

[kpcyrd]

This does the following things:

• To test for cargo features you'd usually use if cfg!(feature = "zlib-ng-compat") { ... } instead of accessing the environment variable, but since this environment variable could also be set from outside of cargo I'm generating cargo:rerun-if-env-changed= output to let cargo know it needs to rebuild if they're changed
• I'm adding a LIBGIT2_NO_VENDOR environment variable, this variable is supposed to deny vendoring even if the cargo feature libgit2-sys/vendored is enabled by any crate. If the system libgit2 is not present or not compatible it errors the build to let the maintainer know something needs to be fixed. This option would be especially useful for Linux package maintainers who always want to link the system library to avoid "shadow copies" of libgit2 embedded in binaries that are not known to the security team.
• Fix one clippy warning

The LIBGIT2_NO_VENDOR variable is analogous to OPENSSL_NO_VENDOR in openssl-sys: https://github.com/sfackler/rust-openssl/pull/1253

There's currently no Error struct in the build.rs so I'm using a Result<(), ()>, if you prefer anything else just let me know.

[classabbyamp]

This would be very useful to void linux, as we've had issues with things not linking to our system libgit2, +1

[lovesegfault]

This needs a rebase

[kpcyrd]

I've rebased the patch

[ehuss]

Closing as completed from #966.