Don't use pkg-config to learn about libgit2 deps

This doesn't work on windows, doesn't work on cross compiles, is a little flaky
in distributions for linking statically, and we need to already do weird things
to not mention libssh2.

Let's keep track of dependencies manually ourselves and express them via the
crates.io resolution graph

libgit2-sys/build.rs

@@ -105,20 +105,6 @@ fn main() {
         return
     }
 
-    if env::var("HOST") == env::var("TARGET") {
-        // libssh2 is linked in elsehwere, don't want it reported via pkg-config
-        let pc = dst.join("lib/pkgconfig/libgit2.pc");
-        let mut contents = String::new();
-        t!(t!(File::open(&pc)).read_to_string(&mut contents));
-        let contents = contents.replace(" -lssh2 ", " ");
-        t!(t!(File::create(&pc)).write_all(contents.as_bytes()));
-
-        prepend("PKG_CONFIG_PATH", dst.join("lib/pkgconfig"));
-        if pkg_config::Config::new().statik(true).find("libgit2").is_ok() {
-            return
-        }
-    }
-
     println!("cargo:rustc-link-lib=static=git2");
     println!("cargo:rustc-link-search=native={}", dst.join("lib").display());
     if target.contains("apple") {