Add defines for libc++ revisions

Since libc++{abi} are included using -isystem and we compile with -MMD (not
-MD), headers from these libraries will not be listed as dependencies.  It's
possible for libc++{abi} to be updated and for ninja to belive no rebuild is
necessary.  This CL adds defines for libc++{abi} to force a rebuild when the
headers are updated.

This CL also requires:
https://chromium-review.googlesource.com/c/chromium/buildtools/+/1002869

R=thakis

Change-Id: I4c48aa18643cd5fee1be23b6379569123e6fef51
Reviewed-on: https://chromium-review.googlesource.com/1002868
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550092}

DEPS

@@ -98,7 +98,7 @@ vars = {
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling build tools
   # and whatever else without interference from each other.
-  'buildtools_revision': 'e8aa02ea839e087f2db66100d02c3b5d47993852',
+  'buildtools_revision': '8febfea9bc7e7d9a7c6105f06f18f7f0e50cfef9',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling SwiftShader
   # and whatever else without interference from each other.

build/config/posix/BUILD.gn

@@ -2,6 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//buildtools/deps_revisions.gni")
 import("//build/config/c++/c++.gni")
 import("//build/config/clang/clang.gni")
 import("//build/config/sanitizers/sanitizers.gni")
@@ -49,6 +50,10 @@ config("runtime_library") {
       "-isystem" + rebase_path("$libcxx_prefix/include", root_build_dir),
       "-isystem" + rebase_path("$libcxxabi_prefix/include", root_build_dir),
     ]
+    defines += [
+      "CR_LIBCXX_REVISION=$libcxx_svn_revision",
+      "CR_LIBCXXABI_REVISION=$libcxxabi_svn_revision",
+    ]
 
     # Make sure we don't link against libc++ or libstdc++.
     if (is_clang) {