Avoid unncessary cargo rebuilds in non-perf configuration

build.rs

@@ -1,7 +1,18 @@
 use std::env;
+use std::fs;
 
 fn main() {
     println!("cargo:rerun-if-changed=target/perf-libs");
+    println!("cargo:rerun-if-changed=build.rs");
+
+    // Ensure target/perf-libs/ exists.  It's been observed that
+    // a cargo:rerun-if-changed= directive with a non-existent
+    // directory triggers a rebuild on every |cargo build| invocation
+    fs::create_dir("target/perf-libs").unwrap_or_else(|err| {
+        if err.kind() != std::io::ErrorKind::AlreadyExists {
+            panic!("Unable to create target/perf-libs: {:?}", err);
+        }
+    });
 
     let cuda = !env::var("CARGO_FEATURE_CUDA").is_err();
     let erasure = !env::var("CARGO_FEATURE_ERASURE").is_err();