Add default empty compiler flags list

Change-Id: Id903db4aec850cd602165df77a1ea060e436d7be
diff --git a/BUILD.gn b/BUILD.gn
index 357e358..93a76eb 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -5,6 +5,8 @@
 config("wuffs_public") {
   include_dirs = [ "release/c" ]
 
+  cflags = []
+
   if (is_clang) {
     cflags += [
       "-Wno-unused-function",