Move pthread flags out compiler config.

That way, buildtools/third_party/libc++/BUILD.gn can opt out of it,
fixing a TODO and a link-time warning when linking libc++.  Once that's
done, we can enable warnings-as-errors for links in asan builds.
(Currently, warnings-as-errors-for-links is enabled everywhere except
asan and chromecast builds.)

No intended behavior change.

BUG=669072
TBR=brettw

Review-Url: https://codereview.chromium.org/2537493002
Cr-Original-Commit-Position: refs/heads/master@{#435255}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 5b58fafbaa929d521e638c16ff638a433ae63f2f

config/BUILDCONFIG.gn

@@ -471,6 +471,7 @@ default_compiler_configs = [
   "//build/config:feature_flags",
   "//build/config/compiler:afdo",
   "//build/config/compiler:compiler",
+  "//build/config/compiler:pthread",
   "//build/config/compiler:clang_stackrealign",
   "//build/config/compiler:compiler_arm_fpu",
   "//build/config/compiler:compiler_arm_thumb",

config/compiler/BUILD.gn

@@ -503,6 +503,16 @@ config("compiler") {
   }
 }
 
+# This is separate from :compiler (and not even a sub-config there)
+# so that some targets can remove it from the list with:
+#   configs -= [ "//build/config/compiler:pthread" ]
+config("pthread") {
+  if (is_linux) {
+    cflags = [ "-pthread" ]
+    ldflags = [ "-pthread" ]
+  }
+}
+
 # This provides the basic options to select the target CPU and ABI.
 # It is factored out of "compiler" so that special cases can use this
 # without using everything that "compiler" brings in.  Options that

config/linux/BUILD.gn

@@ -13,8 +13,6 @@ group("linux") {
 # is applied to all targets. It is here to separate out the logic that is
 # Linux-only. This is not applied to Android, but is applied to ChromeOS.
 config("compiler") {
-  cflags = [ "-pthread" ]
-  ldflags = [ "-pthread" ]
 }
 
 # This is included by reference in the //build/config/compiler:runtime_library