[Mac/GN] Set up the component build.

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/ios/ios_sdk.gni")
@@ skipping 120 matching lines @@
       # the existing .TOC file, overwrite it, otherwise, don't change it.
       #
       # As a special case, if the library reexports symbols from other dynamic
       # libraries, we always update the .TOC and skip the temporary file and
       # diffing steps, since that library always needs to be re-linked.
       tocname = dylib + ".TOC"
       temporary_tocname = dylib + ".tmp"
 
       does_reexport_command = "[ ! -e \"$dylib\" -o ! -e \"$tocname\" ] || otool -l \"$dylib\" | grep -q LC_REEXPORT_DYLIB"
 
-      link_command =
-          "$ld -shared {{ldflags}} -o \"$dylib\" -Wl,-filelist,\"$rspfile\""
+      link_command = "$ld -shared "

[Robert Sesek, 2016/05/06 19:17:36]

This is the hack I mentioned in the standup:

GN doesn't have any concept of install name vs output name, nor can you
filter/check set ldflags. This is an issue with bundled dylib targets, where the
install name differs from the output name, and the install name has to be set by
the target itself. The solution here places the generic
is_component_build-generated install name flag ahead of ldflags, so that if the
target specifies the install name in ldflags, the latter one on the command line
takes precedence.

       if (is_component_build) {
-        link_command += " -Wl,-install_name,@rpath/{{target_output_name}}{{output_extension}}"
+        link_command += " -Wl,-install_name,@rpath/\"{{target_output_name}}{{output_extension}}\" "
       }
-      link_command += " {{solibs}} {{libs}}"
+      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"
 
       rspfile_content = "{{inputs_newline}}"
 
       description = "SOLINK {{output}}"
 
@@ skipping 203 matching lines @@
 }
 
 mac_toolchain("x64") {
   toolchain_cpu = "x64"
   toolchain_os = "mac"
   cc = "${goma_prefix}/gcc"
   cxx = "${goma_prefix}/g++"
   ld = cxx
   is_clang = false
 }