Set --cfg=travis in test/build.rs

Author: geofft

nix-test/build.rs

@@ -13,6 +13,12 @@ pub fn main() {
         "UNKNOWN"
     };
 
+    if let Ok(s) = env::var("TRAVIS") {
+        if s == "true" {
+            println!("cargo:rustc-cfg=travis");
+        }
+    }
+
     gcc::Config::new()
         .file("src/const.c")
         .file("src/sizes.c")