[gn build] port e586607

Made necessary by c479e0c, which requires std::timespec
to exist, which it only does in c++17 and later.

llvm/utils/gn/secondary/libcxx/src/BUILD.gn

@@ -53,7 +53,10 @@ config("cxx_config") {
     "-Wno-user-defined-literals",
     "-Wno-covered-switch-default",
   ]
-  cflags_cc = [ "-nostdinc++" ]
+  cflags_cc = [
+    "-std=c++17",
+    "-nostdinc++",
+  ]
   defines = [ "_LIBCPP_BUILDING_LIBRARY" ]
   if (target_os == "win") {
     cflags += [ "/Zl" ]