Replacing hard-coded clang version by a common variable.

build/toolchain/toolchain.gni

 # Copyright 2015 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.
 
 # Toolchain-related configuration that may be needed outside the context of the
 # toolchain() rules themselves.
 
 declare_args() {
   # Enable Link Time Optimization in optimized builds (output programs run
   # faster, but linking is up to 5-20x slower).
   #
   # TODO(pcc): Remove this flag if/when LTO is enabled in official builds.
   allow_posix_link_time_opt = false
 
   # Set to true to use lld, the LLVM linker. This flag may be used on Windows
   # with the shipped LLVM toolchain, or on Linux with a self-built top-of-tree
   # LLVM toolchain (see llvm_force_head_revision in
   # build/config/compiler/BUILD.gn).
   use_lld = false
+
+  # Clang compiler version. Clang files are placed at version-dependent paths.

[brettw, 2016/05/03 18:03:25]

I was thinking:
  declare_args() {
    ...

    if (is_clang) {
      # Clang compiler version. Clang files are placed at version-dependent
paths.
      clang_version = "3.9.0"
    }
  }

So this won't even be declared as a build arg, or defined at all, unless you're
actually using clang.

[aizatsky, 2016/05/03 18:07:31]

Done. I didn't know conditionals are allowed in declare_args.

+  # It is set to a specific value below only when is_clang==true.
+  clang_version = "undefined"
+}
+
+if (is_clang) {
+  clang_version = "3.9.0"
 }
 
 # Subdirectory within root_out_dir for shared library files.
 # TODO(agrieve): GYP sets this to "lib" for Linux & Android, but this won't work
 #     in GN until support for loadable_module() is added.
 #     See: https://codereview.chromium.org/1236503002/
 shlib_subdir = "."
 
 # Root out dir for shared library files.
 root_shlib_dir = root_out_dir
@@ skipping 32 matching lines @@
 stamp_description = "STAMP {{output}}"
 copy_description = "COPY {{source}} {{output}}"
 if (host_os == "win") {
   stamp_command = "$python_path gyp-win-tool stamp {{output}}"
   copy_command =
       "$python_path gyp-win-tool recursive-mirror {{source}} {{output}}"
 } else {
   stamp_command = "touch {{output}}"
   copy_command = "ln -f {{source}} {{output}} 2>/dev/null || (rm -rf {{output}} && cp -af {{source}} {{output}})"
 }