Un-nest configs in GN files.

third_party/libpng/BUILD.gn

Index: third_party/libpng/BUILD.gn
diff --git a/third_party/libpng/BUILD.gn b/third_party/libpng/BUILD.gn
index c4e2650c8ae6ab2a1e51d4d2ee9f43cc1126e4f7..21420596db7fcaae359799e594aec3d1e320947f 100644
--- a/third_party/libpng/BUILD.gn
+++ b/third_party/libpng/BUILD.gn
@@ -25,6 +25,15 @@ config("libpng_config") {
   }
 }
 
+# Must be in a config because of how GN orders flags (otherwise -Wall will
+# appear after this, and turn it back on).
+config("clang_warnings") {
+  if (is_clang) {
+    # Upstream uses self-assignment to avoid warnings.
+    cflags = [ "-Wno-self-assign" ]
+  }
+}
+
 source_set("libpng_sources") {
   sources = [
     "png.c",
@@ -62,14 +71,6 @@ source_set("libpng_sources") {
     "//third_party/zlib",
   ]
 
-  # Must be in a config because of how GN orders flags (otherwise -Wall will
-  # appear after this, and turn it back on).
-  config("clang_warnings") {
-    if (is_clang) {
-      # Upstream uses self-assignment to avoid warnings.
-      cflags = [ "-Wno-self-assign" ]
-    }
-  }
   configs += [ ":clang_warnings" ]
 }