Make git2 optional

Fixes #3072

Cargo.toml

@@ -38,7 +38,8 @@ binaries = [
     "av-metrics",
     "nom",
 ]
-default = ["binaries", "asm", "threading", "signal_support"]
+default = ["binaries", "asm", "threading", "signal_support", "git_version"]
+git_version = ["built/git2"]
 asm = ["nasm-rs", "cc"]
 threading = ["rayon/threads"]
 signal_support = ["signal-hook"]
@@ -118,7 +119,7 @@ features = ["png"]
 [build-dependencies]
 cc = { version = "1.0", optional = true, features = ["parallel"] }
 rustc_version = "0.4"
-built = { version = "0.5.1", features = ["git2", "chrono"] }
+built = { version = "0.5.1", features = ["chrono"] }
 
 [build-dependencies.nasm-rs]
 version = "0.2"

src/lib.rs

@@ -407,14 +407,33 @@ pub mod version {
     }
   }
 
+  cfg_if::cfg_if! {
+    if #[cfg(feature="git_version")] {
+      fn git_version() -> &'static str {
+        crate::built_info::GIT_VERSION.unwrap_or_default()
+      }
+
+      fn git_hash() -> &'static str {
+        crate::built_info::GIT_COMMIT_HASH.unwrap_or_default()
+      }
+    } else {
+      fn git_version() -> &'static str {
+        "UNKNOWN"
+      }
+
+      fn git_hash() -> &'static str {
+        "UNKNOWN"
+      }
+    }
+  }
   /// Commit hash (short)
   ///
   /// Short hash of the git commit used by this build
   ///
   /// e.g. `g743d464`
   ///
   pub fn hash() -> String {
-    crate::built_info::GIT_COMMIT_HASH.unwrap_or_default().to_string()
+    git_hash().to_string()
   }
 
   /// Version information with the information
@@ -423,11 +442,7 @@ pub mod version {
   /// e.g. `0.1.0 (v0.1.0-1-g743d464)`
   ///
   pub fn full() -> String {
-    format!(
-      "{} ({})",
-      short(),
-      crate::built_info::GIT_VERSION.unwrap_or_default()
-    )
+    format!("{} ({})", short(), git_version(),)
   }
 }
 #[cfg(all(