Ignore unknown warning flags with clang

Make clang ignore unknown warning flags in order to support older
versions of the compiler.

Change-Id: Icff0345e63623d171ec778f5c830b8e0dd7cc433

build/BUILD.gn

diff --git a/build/BUILD.gn b/build/BUILD.gn
index 6e0136f..4078384 100644
--- a/build/BUILD.gn
+++ b/build/BUILD.gn
@@ -25,13 +25,14 @@
   if (is_clang) {
     cflags += [
       "-Weverything",
-      "-Wno-c++98-compat-pedantic",
       "-Wno-c++98-compat",
+      "-Wno-c++98-compat-pedantic",
       "-Wno-gnu-include-next",
       "-Wno-gnu-statement-expression",
       "-Wno-padded",
       "-Wno-reserved-id-macro",
       "-Wno-unused-parameter",
+      "-Wno-unknown-warning-option",
     ]
   }
 }