Add -Z dep-info-omit-d-target to control dep-info style

This avoids a breaking change to dep-info output, putting the
gcc/clang-compliant dep-info behavior behind a flag

src/librustc/session/config.rs

@@ -1236,6 +1236,9 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
          overriding the default for the current target"),
     human_readable_cgu_names: bool = (false, parse_bool, [TRACKED],
         "generate human-readable, predictable names for codegen units"),
+    dep_info_omit_d_target: bool = (false, parse_bool, [TRACKED],
+        "in dep-info output, omit targets for tracking dependencies of the dep-info files \
+         themselves"),
 }
 
 pub fn default_lib_output() -> CrateType {

src/librustc_driver/driver.rs

@@ -1162,8 +1162,9 @@ fn write_out_deps(sess: &Session, outputs: &OutputFilenames, crate_name: &str) {
                     out_filenames.push(p);
                 }
             }
-            // Do not include the dep-info file in itself as a target
-            OutputType::DepInfo => { }
+            OutputType::DepInfo if sess.opts.debugging_opts.dep_info_omit_d_target => {
+                // Don't add the dep-info output when omitting it from dep-info targets
+            }
             _ => {
                 out_filenames.push(file);
             }