Fix unexpected_cfgs lints with the latest rustc

The latest rustc attempts to detect invalid `#[cfg()]` values.  But some
of Mockall's tests conditionalize code on `target_os = "multics"` as an
intentionally always-false condition.  Suppress the lint for those
files.  Also, one CI test sets a custom "reprocheck" variable.  Add that
to the list of known variables.

Author: asomers

mockall/tests/automock_attrs.rs

@@ -1,6 +1,7 @@
 // vim: tw=80
 //! Attributes are applied to the mock object, too.
 #![deny(warnings)]
+#![allow(unexpected_cfgs)] // multics is deliberately always false
 
 use mockall::*;
 

mockall/tests/cfg_attr_concretize.rs

@@ -1,14 +1,15 @@
 // vim: tw=80
 //! #[concretize] can be used inside of #[cfg_attr()]`
 #![deny(warnings)]
+#![allow(unexpected_cfgs)] // multics is deliberately always false
 
 use std::path::{Path, PathBuf};
 
 use mockall::{automock, concretize};
 
 #[automock]
 trait Foo {
-    #[cfg_attr(not(target_os = "ia64-unknown-multics"), concretize)]
+    #[cfg_attr(not(target_os = "multics"), concretize)]
     fn foo<P: AsRef<Path>>(&self, p: P);
 }
 

mockall_derive/build.rs

@@ -0,0 +1,7 @@
+fn main() {
+    // Avoid unnecessary re-building.
+    println!("cargo:rerun-if-changed=build.rs");
+
+    // Quiet rustc's helpful lint.  reprocheck is defined in CI.
+    println!("cargo::rustc-check-cfg=cfg(reprocheck)");
+}