make Valgrind optional

Cargo.toml

@@ -8,5 +8,6 @@ build = "build.rs"
 gcc = "0.3.3"
 
 [features]
-default = ["os"]
+default = ["os", "valgrind"]
 os = []
+valgrind = []

build.rs

@@ -1,5 +1,8 @@
 extern crate gcc;
+use std::env::var_os;
 
 fn main() {
-  gcc::compile_library("libvalgrind.a", &["src/debug/valgrind.c"]);
+  if var_os("CARGO_FEATURE_VALGRIND").is_some() {
+    gcc::compile_library("libvalgrind.a", &["src/debug/valgrind/native.c"]);
+  }
 }

src/debug/valgrind/mod.rs

@@ -0,0 +1,14 @@
+pub use self::imp::*;
+
+#[cfg(feature = "valgrind")]
+#[path = "native.rs"]
+mod imp;
+
+#[cfg(not(feature = "valgrind"))]
+mod imp {
+  //! Stub for the Valgrind functions
+  #![allow(non_camel_case_types)]
+  pub type stack_id_t = ();
+  pub unsafe fn stack_register(_start: *const u8, _end: *const u8) -> stack_id_t {}
+  pub unsafe fn stack_deregister(_id: stack_id_t) {}
+}