Add a variable use_system_xcode to GN.

build/toolchain/mac/BUILD.gn

Index: build/toolchain/mac/BUILD.gn
diff --git a/build/toolchain/mac/BUILD.gn b/build/toolchain/mac/BUILD.gn
index 0c00e995fffe139ac39badd12790acbad3a78a7a..2bb96c75cbeb333f20ab89a7243874fedc99099c 100644
--- a/build/toolchain/mac/BUILD.gn
+++ b/build/toolchain/mac/BUILD.gn
@@ -62,6 +62,12 @@ tool_versions =
 # 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; "

[Nico, 2016/10/04 20:39:33]

why not pass this as an arg to the libtool / ld wrapper script?

...oh, looks like we have an ld wrapper script on linux/android but not on os x?
why is this different? the linkers are almost identical. do we want an ld
wrapper script here?

Come to think of it, does the env var do anything but tell the ld trampoline
which real ld binary to invoke? Could we just call the right ld binary directory
if !use_system_xcode?

[erikchen, 2016/10/05 19:57:19]

Note that sometimes we use linker_driver.py, which in turn may invoke 3
different xcode binaries: dsymutil, strip and ld. Is there a particular reason
you want to see us specify absolute paths to binaries [which relies on the
assumption that these binaries don't call out to other xcode binaries, which I
haven't checked] rather than setting a DEVELOPER_DIR? The latter seems a lot
simpler.

[Nico, 2016/10/05 20:00:44]

Mostly "less magic" -- we'd understand better what's actually happening.

+    }
+
     # 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),
@@ -212,7 +218,7 @@ template("mac_toolchain") {
     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 $script libtool -static {{arflags}} -o {{output}} {{inputs}}"
       description = "LIBTOOL-STATIC {{output}}"
       outputs = [
         "{{output_dir}}/{{target_output_name}}{{output_extension}}",
@@ -250,7 +256,7 @@ template("mac_toolchain") {
       replace_command = "if ! cmp -s \"$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}}"
 
@@ -291,7 +297,7 @@ template("mac_toolchain") {
       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,\"$rspfile\""
       if (is_component_build) {
         link_command += " -Wl,-install_name,@rpath/{{target_output_name}}{{output_extension}}"
       }
@@ -334,7 +340,7 @@ template("mac_toolchain") {
       # do for command-line arguments. Thus any source names with spaces, or
       # label names with spaces (which GN bases the output paths on) will be
       # corrupted by this process. Don't use spaces for source files or labels.
-      command = "$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 $outfile"
       rspfile_content = "{{inputs_newline}}"
       outputs = [
@@ -400,7 +406,7 @@ template("mac_toolchain") {
         _sdk_name = mac_sdk_name
         _min_deployment_target = mac_deployment_target
       }
-      command = "rm -f {{output}} && " +
+      command = "$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}}"