Explicitly pass -Wno-unused-function to the targets that need it.

third_party/yasm/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # The yasm build process creates a slew of small C subprograms that
 # dynamically generate files at various point in the build process.  This makes
 # the build integration moderately complex.
 #
 # There are three classes of dynamically generated files:
 #   1) C source files that should be included in the build (eg., lc3bid.c)
@@ skipping 132 matching lines @@
       "source/patched-yasm/tools/re2c/main.c",
       "source/patched-yasm/tools/re2c/mbo_getopt.c",
       "source/patched-yasm/tools/re2c/parser.c",
       "source/patched-yasm/tools/re2c/scanner.c",
       "source/patched-yasm/tools/re2c/substr.c",
       "source/patched-yasm/tools/re2c/translate.c",
     ]
 
     config("re2c_warnings") {
       # re2c is missing CLOSEVOP from one switch.
-      if (is_posix || is_clang) {
-        cflags = [ "-Wno-switch" ]
+      if (is_clang) {
+        cflags = [
+          # re2c is missing CLOSEVOP from one switch.
+          "-Wno-switch",
+
+          # re2c contains many static functions in headers (because it's
+          # a C library predating C99.)
+          "-Wno-unused-function",
+        ]
       }
     }
-
     configs -= [ "//build/config/compiler:chromium_code" ]
     configs += [
       ":yasm_config",
       "//build/config/compiler:no_chromium_code",
       ":re2c_warnings",
     ]
     deps = [
       "//build/config/sanitizers:deps",
     ]
   }
@@ skipping 323 matching lines @@
 
     # Output eventually #included by source/patched-yasm/frontends/yasm/x86id.c
     outputs = [
       "$yasm_gen_include_dir/x86insns.c",
       "$yasm_gen_include_dir/x86insn_gas.gperf",
       "$yasm_gen_include_dir/x86insn_nasm.gperf",
     ]
     args = [ rebase_path(yasm_gen_include_dir, root_build_dir) ]
   }
 }