Config changes to support target_os="fuchsia"

build/config/fuchsia/BUILD.gn

+# Copyright 2017 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/sysroot.gni")
+
+assert(is_fuchsia)
+assert(is_posix)
+
+config("compiler") {
+  cflags = []
+  cflags_c = [ "-std=c11" ]
+  cflags_cc = [ "-std=c++14" ]

[Nico, 2017/05/11 17:43:37]

Hm, I'd pretty strongly prefer if we could use the same language standard
everywhere. So far we've been able to do that – and given that the goal is to
build base/, that should be enough for chromium too, right?

On that note, which c++ standard library are you going to use? We're working on
switching linux to libc++; is fuchsia on libstd++ or libc++? And is your C
library glibc, musl, or something else?

[scottmg, 2017/05/11 17:52:53]

OK, I was thinking it would match the OS headers, but it probably doesn't matter
in any case at least for the c++ setting. I could more imagine c11 being a
problem (or maybe that's the default anyway?), but I guess we can remove it, and
then complain upstream to get headers that work with whatever our default is, if
it becomes a problem.
libc++ and musl afaik.

+  ldflags = [ "-Wl,--threads" ]
+  configs = [
+    ":compiler_sysroot",
+    ":compiler_target",
+  ]
+  asmflags = cflags + cflags_c
+
+  libs = [
+    "mxio",
+    "magenta",
+    "unwind",
+  ]
+}
+
+config("compiler_sysroot") {
+  # The sole purpose of SYSROOT_VERSION is to change the command line on every
+  # sysroot update so as to force rebuilds.
+  defines = [ "SYSROOT_VERSION=$sysroot_version" ]
+  cflags = [ "--sysroot=" + rebase_path(sysroot, root_build_dir) ]
+  ldflags = [ "--sysroot=" + rebase_path(sysroot, root_build_dir) ]
+  asmflags = cflags
+}
+
+config("compiler_target") {
+  cflags = []
+  ldflags = []
+  if (current_cpu == "arm64") {
+    cflags += [ "--target=aarch64-fuchsia" ]
+    ldflags += [ "--target=aarch64-fuchsia" ]
+  } else if (current_cpu == "x64") {
+    cflags += [ "--target=x86_64-fuchsia" ]
+    ldflags += [ "--target=x86_64-fuchsia" ]
+  } else {
+    assert(false, "Unsupported architecture")
+  }
+  asmflags = cflags
+}