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

third_party/libxslt/BUILD.gn

Index: third_party/libxslt/BUILD.gn
diff --git a/third_party/libxslt/BUILD.gn b/third_party/libxslt/BUILD.gn
index 24325b03c408de0ef07b9bba06c60f25abd1f60d..6b46db384f56dda1354718f69fa56b5b74767a87 100644
--- a/third_party/libxslt/BUILD.gn
+++ b/third_party/libxslt/BUILD.gn
@@ -59,8 +59,21 @@ static_library("libxslt") {
     "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 = []
@@ -73,10 +86,6 @@ static_library("libxslt") {
     include_dirs = [ "mac" ]
   }
 
-  if (is_clang) {
-    cflags += [ "-Wno-pointer-sign" ]
-  }
-
   deps = [
     "//third_party/libxml",
   ]