Only run the build script if something changed.

I agree to license my contributions to each file under the terms
given at the top of each file I changed.

build.rs

@@ -446,6 +446,7 @@ fn build_unix(target_info: &TargetInfo, out_dir: PathBuf) {
         "stdc++"
     };
     println!("cargo:rustc-flags=-l dylib={}", libcxx);
+    print_rerun();
 }
 
 
@@ -686,3 +687,22 @@ fn walk_dir<F>(dir: &Path, cb: &F)
         }
     }
 }
+
+fn print_rerun() {
+    for s in RING_ARM_SHARED_SRCS.iter()
+        .chain(RING_SRC.iter())
+        .chain(RING_TEST_SRCS.iter())
+        .chain(RING_AARCH64_SRCS.iter())
+        .chain(RING_ARM_SRCS.iter())
+        .chain(RING_X86_64_SRC.iter())
+        .chain(RING_X86_SRCS.iter())
+        .chain(RING_INTEL_SHARED_SRCS.iter())
+        .chain(RING_PPC_SRCS.iter())
+        .chain(RING_HEADERS.iter())
+        .chain(RING_TEST_HEADERS.iter())
+        .chain(RING_PERL_INCLUDES.iter())
+        .chain(RING_INLINE_FILES.iter()) {
+        println!("cargo:rerun-if-changed={}", s);
+    }
+    println!("cargo:rerun-if-changed=build.rs");
+}