Actually add the rust side of the test

gcc-test/build.rs

@@ -69,4 +69,8 @@ fn main() {
         println!("cargo:rustc-link-lib=msvc");
         println!("cargo:rustc-link-search={}", out.display());
     }
+
+    // This tests whether we  can build a library but not link it to the main crate.
+    // The test module will do its own linking.
+    gcc::Config::new().link(false).file("src/opt_linkage.c").compile("libOptLinkage.a");
 }

gcc-test/tests/all.rs

@@ -2,6 +2,14 @@ extern crate gcc_test;
 
 use gcc_test::*;
 
+
+// This will cause a link failure if the archive is already
+// linked against the gcc_test crate.
+#[link(name = "OptLinkage", kind = "static")]
+extern {
+  fn answer() -> i32;
+}
+
 #[test]
 fn foo_here() {
     unsafe {
@@ -46,3 +54,8 @@ fn msvc_here() {
         msvc();
     }
 }
+
+#[test]
+fn opt_linkage() {
+   unsafe { assert_eq!(answer(),42); }
+}