Auto merge of #3180 - jsha:print-rustc-error-output, r=JohnTitor

build.rs: print rustc stderr if exit status != 0

I was trying to run benchmarks locally with rustc-perf and found that many of them failed to build with a message from libc's build.rs "Failed to get rustc version." I made this change locally to help debug, and I think it would be generally useful. In my case it quickly revealed that rustc was failing to find libLLVM and so `rustc --version` was emitting nothing on stdout.

I think this may have been part of what was intended with #3000 and might help debug rust-lang/crater#663.

build.rs

@@ -1,6 +1,7 @@
 use std::env;
 use std::process::Command;
 use std::str;
+use std::string::String;
 
 // List of cfgs this build script is allowed to set. The list is needed to support check-cfg, as we
 // need to know all the possible cfgs that this script will set. If you need to set another cfg
@@ -181,6 +182,13 @@ fn rustc_minor_nightly() -> (u32, bool) {
         .output()
         .ok()
         .expect("Failed to get rustc version");
+    if !output.status.success() {
+        panic!(
+            "failed to run rustc: {}",
+            String::from_utf8_lossy(output.stderr.as_slice())
+        );
+    }
+
     let version = otry!(str::from_utf8(&output.stdout).ok());
     let mut pieces = version.split('.');