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

third_party/libxslt/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.
 
 config("libxslt_config") {
   defines = [ "LIBXSLT_STATIC" ]
   include_dirs = [ "." ]
 }
 
 static_library("libxslt") {
@@ skipping 41 matching lines @@
     "libxslt/xsltlocale.c",
     "libxslt/xsltlocale.h",
     "libxslt/xsltutils.c",
     "libxslt/xsltutils.h",
     "libxslt/xsltwin32config.h",
     "linux/config.h",
     "mac/config.h",
     "win32/config.h",
   ]
 
+  config("libxslt_warnings") {
+    if (is_clang) {
+      cflags = [
+        # libxslt stores a char[3] in a `const unsigned char*`.
+        "-Wno-pointer-sign",
+
+        # xsltDefaultRegion and xsltCalibrateTimestamps are only
+        # used with certain preprocessor defines set.
+        "-Wno-unused-function",
+      ]
+    }
+  }
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [ "//build/config/compiler:no_chromium_code" ]
+  configs += [ ":libxslt_warnings" ]
   public_configs = [ ":libxslt_config" ]
 
   cflags = []
   if (is_linux || is_android) {
     include_dirs = [ "linux" ]
   } else if (is_win) {
     include_dirs = [ "win32" ]
     cflags += [ "/wd4267" ]  # size_t to int.
   } else if (is_mac) {
     include_dirs = [ "mac" ]
   }
 
-  if (is_clang) {
-    cflags += [ "-Wno-pointer-sign" ]
-  }
-
   deps = [
     "//third_party/libxml",
   ]
 }