rustbuild: Skip generating docs if the config disables them.

It looks like before these config variables weren't actually taken
into account. This patch should make the build system skip over the
documentation steps correctly.

Author: timonvo

src/bootstrap/build/step.rs

@@ -148,7 +148,9 @@ fn top_level(build: &Build) -> Vec<Step> {
             src: Source::Llvm { _dummy: () },
             target: &build.config.build,
         };
-        targets.push(t.doc(stage));
+        if build.config.docs {
+          targets.push(t.doc(stage));
+        }
         for host in build.config.host.iter() {
             if !build.flags.host.contains(host) {
                 continue
@@ -350,7 +352,9 @@ impl<'a> Step<'a> {
                     let compiler = self.compiler(stage);
                     for target in build.config.target.iter() {
                         let target = self.target(target);
-                        base.push(target.dist_docs(stage));
+                        if build.config.docs {
+                            base.push(target.dist_docs(stage));
+                        }
                         base.push(target.dist_std(compiler));
                     }
                 }