Mov Linux & Mac GN compiler flags to separate configs.

build/config/linux/BUILD.gn

Index: build/config/linux/BUILD.gn
diff --git a/build/config/linux/BUILD.gn b/build/config/linux/BUILD.gn
index e39155c611472fd6d97bc5c2d228f7f70ed98362..34a7f749ca0eb574ae738c75e5fa78e97ae3fe55 100644
--- a/build/config/linux/BUILD.gn
+++ b/build/config/linux/BUILD.gn
@@ -7,6 +7,14 @@ import("//build/config/features.gni")
 import("//build/config/sysroot.gni")
 import("//build/config/ui.gni")
 
+# This is included by reference in the //build/config/compiler config that
+# 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
 # config that is applied to all targets. It is here to separate out the logic
 # that is Linux-only. Please see that target for advice on what should go in