Stop using -isystem and -imsvc because "gn check" ignores them.

Gn check can enforce dependencies only if the header is included with
an absolute path (from the root of the project) or if "include_dirs"
is used. It ignores -isystem and -imsvc since they are part of
cflags (and similar variants).

Since //third_party/abseil-cpp/absl:absl_include_config was using
-isystem and -imsvc only to sidestep compiler warnings in absl code,
it is good to try to switch back to include_dirs instead of adding
-isystem/-imsvc support to "gn check".

Bug: 862521
Change-Id: I9c2d01abad75023918a88f852e3080486dcafead
Reviewed-on: https://chromium-review.googlesource.com/1124478
Commit-Queue: Mirko Bonadei <mbonadei@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577535}

third_party/abseil-cpp/BUILD.gn

@@ -19,28 +19,7 @@ group("default") {
 }
 
 config("absl_include_config") {
-  # Using -isystem (with clang and GCC) and -imsvc (with clang-cl) instead of
-  # include_dirs (-I), so we don't need to suppress warnings coming from
-  # Abseil. Doing so would mask warnings in our own code.
-  if (is_win) {
-    if (is_clang) {
-      # clang-cl:
-      cflags = [
-        "-imsvc",
-        rebase_path(".", root_build_dir),
-      ]
-    } else {
-      # MSVC doesn't have -isystem, in that case we fallback to include_dirs and
-      # we use the warning suppression flags defined in :absl_default_cflags_cc.
-      include_dirs = [ "." ]
-    }
-  } else {
-    # GCC or clang:
-    cflags = [
-      "-isystem",
-      rebase_path(".", root_build_dir),
-    ]
-  }
+  include_dirs = [ "." ]
 }
 
 config("absl_define_config") {

third_party/abseil-cpp/absl/time/BUILD.gn

@@ -14,11 +14,21 @@ if (build_with_chromium) {
   visibility = [ "*" ]
 }
 
+config("suppress_unguarded_availability") {
+  # TODO(bugs.webrtc.org/9557): Remove -Wno-unguarded-availability when
+  # abseil will support Xcode 9.0+ (it currently supports Xcode 7.3.1+
+  # which doesn't have -Wunguarded-availability and __builtin_available).
+  cflags = [
+    "-Wno-unguarded-availability",
+  ]
+}
+
 source_set("time") {
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [
     "//build/config/compiler:no_chromium_code",
     "//third_party/abseil-cpp:absl_default_cflags_cc",
+    ":suppress_unguarded_availability",
   ]
   public_configs = [ "//third_party/abseil-cpp:absl_include_config" ]
   sources = [