Add GN output prefix override and allow empty output extensions.

tools/gn/function_toolchain.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 <algorithm>
 #include <limits>
 #include <utility>
 
 #include "tools/gn/err.h"
 #include "tools/gn/functions.h"
@@ skipping 516 matching lines @@
     "\n"
     "    output_prefix  [string]\n"
     "        Valid for: Linker tools (optional)\n"
     "\n"
     "        Prefix to use for the output name. Defaults to empty. This\n"
     "        prefix will be prepended to the name of the target (or the\n"
     "        output_name if one is manually specified for it) if the prefix\n"
     "        is not already there. The result will show up in the\n"
     "        {{output_name}} substitution pattern.\n"
     "\n"
+    "        Individual targets can opt-out of the output prefix by setting:\n"
+    "          output_prefix_override = true\n"
+    "        (see \"gn help output_prefix_override\").\n"
+    "\n"
     "        This is typically used to prepend \"lib\" to libraries on\n"
     "        Posix systems:\n"
     "          output_prefix = \"lib\"\n"
     "\n"
     "    precompiled_header_type  [string]\n"
     "        Valid for: \"cc\", \"cxx\", \"objc\", \"objcxx\"\n"
     "\n"
     "        Type of precompiled headers. If undefined or the empty string,\n"
     "        precompiled headers will not be used for this tool. Otherwise\n"
     "        use \"gcc\" or \"msvc\".\n"
@@ skipping 462 matching lines @@
     return Value();
 
   Scope::KeyValueMap values;
   block_scope.GetCurrentScopeValues(&values);
   toolchain->args() = values;
 
   return Value();
 }
 
 }  // namespace functions