Add option make_clang_dir to GN

build/toolchain/gcc_toolchain.gni

 # 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.
 
 import("//build/config/nacl/config.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/toolchain/ccache.gni")
 import("//build/toolchain/goma.gni")
 import("//build/toolchain/toolchain.gni")
 
@@ skipping 407 matching lines @@
         is_tsan = false
         is_ubsan = false
         is_ubsan_vptr = false
       }
     }
 
     forward_variables_from(invoker, [ "deps" ])
   }
 }
 
+declare_args() {
+  # changes the path to the clang executable
+  make_clang_dir = "default"

[Dirk Pranke, 2016/01/25 20:47:46]

this seems kinda crufty.

Can you just do:

  declare_args() {
    clang_dir = "//third_party/llvm-build/Release+Asserts/bin"
  }

and change line 452 to

  prefix = rebase_path(clang_dir, root_build_dir)

instead?

It's better to use a good variable name than it is to match GYP ...

[Nico, 2016/01/25 21:07:19]

I agree that the current name isn't great. It's historically named after a
feature in gyp's make generator (hence "make_") -- it shouldn't have this name
in gn :-)

I'd suggest defaulting it to //third_party/llvm-build/Release+Asserts/ (no bin/)
though, since some bits will probably need stuff below lib/ too.

[mkollaro, 2016/01/26 09:03:37]

I'm not sure how to do that in a reasonable manner - if I do

  prefix = rebase_path(clang_dir + "/bin", root_build_dir)

then the user who wants to change clang_dir would have to point it to the parent
directory, which is not very intuitive. I'd rather leave it with the "bin"
suffix, because YAGNI.

[mkollaro, 2016/01/26 09:03:37]

Done.

+}
+
 # This is a shorthand for gcc_toolchain instances based on the
 # Chromium-built version of Clang.  Only the toolchain_cpu and
 # toolchain_os variables need to be specified by the invoker, and
 # optionally toolprefix if it's a cross-compile case.  Note that for
 # a cross-compile case this toolchain requires a config to pass the
 # appropriate -target option, or else it will actually just be doing
 # a native compile.  The invoker can optionally override use_gold too.
 template("clang_toolchain") {
   assert(defined(invoker.toolchain_cpu),
          "clang_toolchain() must specify a \"toolchain_cpu\"")
   assert(defined(invoker.toolchain_os),
          "clang_toolchain() must specify a \"toolchain_os\"")
   if (defined(invoker.toolprefix)) {
     toolprefix = invoker.toolprefix
   } else {
     toolprefix = ""
   }
 
   gcc_toolchain(target_name) {
     prefix = rebase_path("//third_party/llvm-build/Release+Asserts/bin",
                          root_build_dir)
+    if (make_clang_dir != "default") {
+      prefix = make_clang_dir
+    }
     cc = "$prefix/clang"
     cxx = "$prefix/clang++"
     ld = cxx
     is_clang = true
 
     readelf = "${toolprefix}readelf"
     ar = "${toolprefix}ar"
     nm = "${toolprefix}nm"
 
     forward_variables_from(invoker,
                            [
                              "toolchain_cpu",
                              "toolchain_os",
                              "use_gold",
                            ])
   }
 }