Auto merge of #383 - Xanewok:rerun-script, r=jdm

Don't rerun build script unnecessarily

Closes #380. From what I can see the only thing [impacting](https://github.com/servo/rust-mozjs/blob/master/CMakeLists.txt#L7) the cmake build is the jsglue.cpp file, so I set to only watch that file, instead.

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build.rs

@@ -8,4 +8,5 @@ fn main() {
     let dst = cmake::Config::new(".").build();
     println!("cargo:rustc-link-search=native={}/lib", dst.display());
     println!("cargo:rustc-link-lib=static=jsglue");
+    println!("cargo:rerun-if-changed=src/jsglue.cpp");
 }