Add an option to disabled automatic linking of the produced binary.

This is mostly useful when your tests require building some piece of C code,
but you do not want the entire crate to be linked against it. You can then
just do `#[cfg_attr(test,link(name = "mylibrary", kind = "static"))]` and the
code will only be linked for tests.

src/lib.rs

@@ -75,6 +75,7 @@ pub struct Config {
     env: Vec<(OsString, OsString)>,
     compiler: Option<PathBuf>,
     archiver: Option<PathBuf>,
+    link: bool,
 }
 
 fn getenv(v: &str) -> Option<String> {
@@ -133,6 +134,7 @@ impl Config {
             env: Vec::new(),
             compiler: None,
             archiver: None,
+            link: true
         }
     }
 
@@ -286,6 +288,16 @@ impl Config {
         self.archiver = Some(archiver.as_ref().to_owned());
         self
     }
+    /// Define whether the compiled archive will automatically be linked.
+    ///
+    /// If you don't want the compiled archive to be linked into the rust
+    /// binary automatically, you can set this option to `false`. It can
+    /// still be linked explicitly using the `#[cfg(link())]` directive.
+    pub fn link(&mut self, link: bool) -> &mut Config {
+        self.link = link;
+        self
+    }
+
 
     #[doc(hidden)]
     pub fn __set_env<A, B>(&mut self, a: A, b: B) -> &mut Config
@@ -313,8 +325,10 @@ impl Config {
 
         self.assemble(lib_name, &dst.join(output), &objects);
         println!("cargo:rustc-link-search=native={}", dst.display());
-        println!("cargo:rustc-link-lib=static={}",
-                 &output[3..output.len() - 2]);
+        if self.link {
+          println!("cargo:rustc-link-lib=static={}",
+                   &output[3..output.len() - 2]);
+        }
 
         // Add specific C++ libraries, if enabled.
         if self.cpp {