build: Avoid unnecessary build script reruns in libstd

Add a FIXME to build scripts in profiler_builtins

src/libprofiler_builtins/build.rs

@@ -9,6 +9,8 @@ fn main() {
     let target = env::var("TARGET").expect("TARGET was not set");
     let cfg = &mut cc::Build::new();
 
+    // FIXME: `rerun-if-changed` directives are not currently emitted and the build script
+    // will not rerun on changes in these source files or headers included into them.
     let mut profile_sources = vec![
         "GCDAProfiling.c",
         "InstrProfiling.c",

src/libstd/build.rs

@@ -1,6 +1,7 @@
 use std::env;
 
 fn main() {
+    println!("cargo:rerun-if-changed=build.rs");
     let target = env::var("TARGET").expect("TARGET was not set");
     if target.contains("linux") {
         if target.contains("android") {