Add a variable use_system_xcode to GN.

build/toolchain/mac/BUILD.gn

 # 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.
 
 # TODO(brettw) Use "gcc_toolchain.gni" like the Linux toolchains. This requires
 # some enhancements since the commands on Mac are slightly different than on
 # Linux.
 
 import("../goma.gni")
 import("//build/config/clang/clang.gni")
@@ skipping 44 matching lines @@
                               "//build/toolchain/mac/filter_libtool.py",
                               "//build/toolchain/mac/linker_driver.py",
                             ],
                             root_build_dir),
                 "trim scope")
 
 # Shared toolchain definition. Invocations should set current_os to set the
 # build args in this definition.
 template("mac_toolchain") {
   toolchain(target_name) {
+    if (use_system_xcode) {
+      env_wrapper = ""
+    } else {
+      env_wrapper = "export DEVELOPER_DIR=$hermetic_xcode_path; "
+    }
+
     # When invoking this toolchain not as the default one, these args will be
     # passed to the build. They are ignored when this is the default toolchain.
     assert(defined(invoker.toolchain_args),
            "Toolchains must declare toolchain_args")
     toolchain_args = {
       # Populate toolchain args from the invoker.
       forward_variables_from(invoker.toolchain_args, "*")
 
       # The host toolchain value computed by the default toolchain's setup
       # needs to be passed through unchanged to all secondary toolchains to
@@ skipping 130 matching lines @@
       depsformat = "gcc"
       description = "OBJCXX {{output}}"
       outputs = [
         "$object_subdir/{{source_name_part}}.o",
       ]
     }
 
     tool("alink") {
       script =
           rebase_path("//build/toolchain/mac/filter_libtool.py", root_build_dir)
-      command = "rm -f {{output}} && TOOL_VERSION=${tool_versions.filter_libtool} python $script libtool -static {{arflags}} -o {{output}} {{inputs}}"
+      command = "$env_wrapper rm -f {{output}} && TOOL_VERSION=${tool_versions.filter_libtool} python $script libtool -static {{arflags}} -o {{output}} {{inputs}}"
       description = "LIBTOOL-STATIC {{output}}"
       outputs = [
         "{{output_dir}}/{{target_output_name}}{{output_extension}}",
       ]
       default_output_dir = "{{target_out_dir}}"
       default_output_extension = ".a"
       output_prefix = "lib"
     }
 
     tool("solink") {
@@ skipping 17 matching lines @@
       link_command = "$linker_driver $ld -shared "
       if (is_component_build) {
         link_command += " -Wl,-install_name,@rpath/\"{{target_output_name}}{{output_extension}}\" "
       }
       link_command += dsym_switch
       link_command += "{{ldflags}} -o \"$dylib\" -Wl,-filelist,\"$rspfile\" {{libs}} {{solibs}}"
 
       replace_command = "if ! cmp -s \"$temporary_tocname\" \"$tocname\"; then mv \"$temporary_tocname\" \"$tocname\""
       extract_toc_command = "{ otool -l \"$dylib\" | grep LC_ID_DYLIB -A 5; nm -gP \"$dylib\" | cut -f1-2 -d' ' | grep -v U\$\$; true; }"
 
-      command = "if $does_reexport_command ; then $link_command && $extract_toc_command > \"$tocname\"; else $link_command && $extract_toc_command > \"$temporary_tocname\" && $replace_command ; fi; fi"
+      command = "$env_wrapper if $does_reexport_command ; then $link_command && $extract_toc_command > \"$tocname\"; else $link_command && $extract_toc_command > \"$temporary_tocname\" && $replace_command ; fi; fi"
 
       rspfile_content = "{{inputs_newline}}"
 
       description = "SOLINK {{output}}"
 
       # Use this for {{output_extension}} expansions unless a target manually
       # overrides it (in which case {{output_extension}} will be what the target
       # specifies).
       default_output_dir = "{{root_out_dir}}"
       default_output_extension = ".dylib"
@@ skipping 20 matching lines @@
       if (_save_unstripped_output) {
         outputs += [ _unstripped_output ]
       }
     }
 
     tool("solink_module") {
       sofile = "{{output_dir}}/{{target_output_name}}{{output_extension}}"  # eg "./libfoo.so"
       rspfile = sofile + ".rsp"
       pool = "//build/toolchain:link_pool($default_toolchain)"
 
-      link_command = "$linker_driver $ld -bundle {{ldflags}} -o \"$sofile\" -Wl,-filelist,\"$rspfile\""
+      link_command = "$env_wrapper $linker_driver $ld -bundle {{ldflags}} -o \"$sofile\" -Wl,-filelist,\"$rspfile\""
       if (is_component_build) {
         link_command += " -Wl,-install_name,@rpath/{{target_output_name}}{{output_extension}}"
       }
       link_command += dsym_switch
       link_command += " {{solibs}} {{libs}}"
       command = link_command
 
       rspfile_content = "{{inputs_newline}}"
 
       description = "SOLINK_MODULE {{output}}"
@@ skipping 22 matching lines @@
       pool = "//build/toolchain:link_pool($default_toolchain)"
 
       # Note about --filelist: Apple's linker reads the file list file and
       # interprets each newline-separated chunk of text as a file name. It
       # doesn't do the things one would expect from the shell like unescaping
       # or handling quotes. In contrast, when Ninja finds a file name with
       # spaces, it single-quotes them in $inputs_newline as it would normally
       # do for command-line arguments. Thus any source names with spaces, or
       # label names with spaces (which GN bases the output paths on) will be
       # corrupted by this process. Don't use spaces for source files or labels.
-      command = "$linker_driver $ld $dsym_switch {{ldflags}} -o \"$outfile\" -Wl,-filelist,\"$rspfile\" {{solibs}} {{libs}}"
+      command = "$env_wrapper $linker_driver $ld $dsym_switch {{ldflags}} -o \"$outfile\" -Wl,-filelist,\"$rspfile\" {{solibs}} {{libs}}"
       description = "LINK $outfile"
       rspfile_content = "{{inputs_newline}}"
       outputs = [
         outfile,
       ]
 
       if (_enable_dsyms) {
         outputs += dsym_output
       }
       if (_save_unstripped_output) {
@@ skipping 45 matching lines @@
     tool("compile_xcassets") {
       _tool = rebase_path("//build/toolchain/mac/compile_xcassets.py",
                           root_build_dir)
       if (is_ios) {
         _sdk_name = ios_sdk_name
         _min_deployment_target = ios_deployment_target
       } else {
         _sdk_name = mac_sdk_name
         _min_deployment_target = mac_deployment_target
       }
-      command = "rm -f {{output}} && " +
+      command = "$env_wrapper rm -f {{output}} && " +
                 "TOOL_VERSION=${tool_versions.compile_xcassets} " +
                 "python $_tool -p $_sdk_name -t $_min_deployment_target " +
                 "-T {{bundle_product_type}} -o {{output}} {{inputs}}"
 
       description = "COMPILE_XCASSETS {{output}}"
       pool = ":bundle_pool($default_toolchain)"
     }
   }
 }
 
@@ skipping 33 matching lines @@
     }
   }
 
   mac_toolchain("ios_clang_x64") {
     toolchain_args = {
       current_cpu = "x64"
       current_os = "ios"
     }
   }
 }