gn: don't reject absolute unix style pathnames in labels

tools/gn/label.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "tools/gn/label.h"
 
 #include "base/logging.h"
 #include "tools/gn/err.h"
 #include "tools/gn/parse_tree.h"
 #include "tools/gn/value.h"
@@ skipping 17 matching lines @@
                                  const SourceDir& current_dir,
                                  const base::StringPiece& input,
                                  SourceDir* result,
                                  Err* err) {
   // No rule, use the current locaton.
   if (input.empty()) {
     *result = current_dir;
     return true;
   }
 
-  // Don't allow directories to start with a single slash. All labels must be
-  // in the source root.
-  if (input[0] == '/' && (input.size() == 1 || input[1] != '/')) {

[tfarina, 2014/12/30 02:38:00]

OK. let's try.

-    *err = Err(input_value, "Label can't start with a single slash",
-        "Labels must be either relative (no slash at the beginning) or be "
-        "absolute\ninside the source root (two slashes at the beginning).");
-    return false;
-  }
-
   *result = current_dir.ResolveRelativeDir(input);
   return true;
 }
 
 // Given the separated-out target name (after the colon) computes the final
 // name, using the implicit name from the previously-generated
 // computed_location if necessary. The input_value is used only for generating
 // error messages.
 bool ComputeTargetNameFromDep(const Value& input_value,
                               const SourceDir& computed_location,
@@ skipping 205 matching lines @@
   }
   return ret;
 }
 
 std::string Label::GetUserVisibleName(const Label& default_toolchain) const {
   bool include_toolchain =
       default_toolchain.dir() != toolchain_dir_ ||
       default_toolchain.name() != toolchain_name_;
   return GetUserVisibleName(include_toolchain);
 }