Add vendored feature (#739)

As per #721, some consumers may
prefer to build `libgit2` themselves to simplify distribution.

To test the vendored version, you can run:

```
cargo build --features vendored-libgit2`
```

That being said, the test `branch::tests::name_is_valid` fails on macOS. This
test is currently broken as per
#721 (comment)

Cargo.toml

@@ -38,6 +38,7 @@ unstable = []
 default = ["ssh", "https", "ssh_key_from_memory"]
 ssh = ["libgit2-sys/ssh"]
 https = ["libgit2-sys/https", "openssl-sys", "openssl-probe"]
+vendored-libgit2 = ["libgit2-sys/vendored"]
 vendored-openssl = ["openssl-sys/vendored", "libgit2-sys/vendored-openssl"]
 ssh_key_from_memory = ["libgit2-sys/ssh_key_from_memory"]
 zlib-ng-compat = ["libgit2-sys/zlib-ng-compat"]

libgit2-sys/Cargo.toml

@@ -36,6 +36,7 @@ openssl-sys = { version = "0.9", optional = true }
 ssh = ["libssh2-sys"]
 https = ["openssl-sys"]
 ssh_key_from_memory = []
+vendored = []
 vendored-openssl = ["openssl-sys/vendored"]
 # Cargo does not support requiring features on an optional dependency without
 # requiring the dependency. Rather than introduce additional complexity, we

libgit2-sys/build.rs

@@ -7,10 +7,12 @@ use std::process::Command;
 fn main() {
     let https = env::var("CARGO_FEATURE_HTTPS").is_ok();
     let ssh = env::var("CARGO_FEATURE_SSH").is_ok();
+    let vendored = env::var("CARGO_FEATURE_VENDORED").is_ok();
     let zlib_ng_compat = env::var("CARGO_FEATURE_ZLIB_NG_COMPAT").is_ok();
 
     // To use zlib-ng in zlib-compat mode, we have to build libgit2 ourselves.
-    if !zlib_ng_compat {
+    let try_to_use_system_libgit2 = !vendored && !zlib_ng_compat;
+    if try_to_use_system_libgit2 {
         let mut cfg = pkg_config::Config::new();
         if let Ok(lib) = cfg.atleast_version("1.1.0").probe("libgit2") {
             for include in &lib.include_paths {