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

third_party/libxml/BUILD.gn

Index: third_party/libxml/BUILD.gn
diff --git a/third_party/libxml/BUILD.gn b/third_party/libxml/BUILD.gn
index 58a5f1c1040d3ae8ad8909da52d7d56c84c4c191..c9676088ac2ff77225ef5a0e4841295b0aa79310 100644
--- a/third_party/libxml/BUILD.gn
+++ b/third_party/libxml/BUILD.gn
@@ -167,33 +167,34 @@ static_library("libxml") {
     defines = [ "_REENTRANT" ]
   }
 
-  if (is_clang) {
-    cflags = [
-      # libxml passes `const unsigned char*` through `const char*`.
-      "-Wno-pointer-sign",
-
-      # pattern.c and uri.c both have an intentional `for (...);` /
-      # `while(...);` loop. I submitted a patch to move the `'` to its own
-      # line, but until that's landed suppress the warning:
-      "-Wno-empty-body",
-
-      # debugXML.c compares array 'arg' to NULL.
-      "-Wno-tautological-pointer-compare",
-
-      # threads.c attempts to forward declare a pthread_equal which doesn't
-      # match the prototype in pthreads.h
-      "-Wno-ignored-attributes",
-    ]
-  }
   config("libxml_warnings") {
     if (is_clang) {
-      cflags += [
+      cflags = [
+        # libxml passes `const unsigned char*` through `const char*`.
+        "-Wno-pointer-sign",
+
+        # pattern.c and uri.c both have an intentional `for (...);` /
+        # `while(...);` loop. I submitted a patch to move the `'` to its own
+        # line, but until that's landed suppress the warning:
+        "-Wno-empty-body",
+
+        # debugXML.c compares array 'arg' to NULL.
+        "-Wno-tautological-pointer-compare",
+
+        # threads.c attempts to forward declare a pthread_equal which doesn't
+        # match the prototype in pthreads.h
+        "-Wno-ignored-attributes",
+
         # libxml casts from int to long to void*.
         "-Wno-int-to-void-pointer-cast",
 
         # libxml passes a volatile LPCRITICAL_SECTION* to a function expecting
         # a void* volatile*.
         "-Wno-incompatible-pointer-types",
+
+        # trio_is_special_quantity and trio_is_negative are only
+        # used with certain preprocessor defines set.
+        "-Wno-unused-function",
       ]
     }
   }