build.rs: silence error output of compile checks

These compile checks are expected to produce compiler errors on some systems.
The errors show up when there is an unrelated error, this is probably quite
confusing so fix that. Should revisit this later.

fish-rust/build.rs

@@ -1,6 +1,7 @@
 use rsconf::{LinkType, Target};
 use std::env;
 use std::error::Error;
+use std::process::Stdio;
 
 fn main() {
     for key in ["DOCDIR", "DATADIR", "SYSCONFDIR", "BINDIR"] {
@@ -169,6 +170,8 @@ fn compiles(file: &str) -> bool {
         .get_compiler()
         .to_command();
     command.arg(file);
+    command.stdout(Stdio::null());
+    command.stderr(Stdio::null());
     command.status().unwrap().success()
 }