Add option make_clang_dir to GN

build/toolchain/gcc_toolchain.gni

Index: build/toolchain/gcc_toolchain.gni
diff --git a/build/toolchain/gcc_toolchain.gni b/build/toolchain/gcc_toolchain.gni
index 38e5fb6d3e97d211417865fe4a6f6f91bfefb8af..07fc2bbee16c31a569c4e22490ad84611413a374 100644
--- a/build/toolchain/gcc_toolchain.gni
+++ b/build/toolchain/gcc_toolchain.gni
@@ -425,6 +425,11 @@ template("gcc_toolchain") {
   }
 }
 
+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
@@ -446,6 +451,9 @@ template("clang_toolchain") {
   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