clang/win: Build without -Wno-incompatible-pointer-types.

It defines YY_USE_CONST when building mesa to fix this warning, disables
-Wincompatible-pointer-types for the other third-party libraries that
trigger it and fixes some underspecified dependencies in
third_party/libwebp.

BUG=504696

Review URL: https://codereview.chromium.org/1226583002

Cr-Original-Commit-Position: refs/heads/master@{#338227}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: c98d446e81bcc18cf4241325c8719ccdc0b7abf3

BUILD.gn

@@ -67,6 +67,13 @@ static_library("zlib") {
 
   if (!is_ios && (current_cpu == "x86" || current_cpu == "x64")) {
     sources += [ "x86.c" ]
+
+    config("zlib_warnings") {
+      if (is_clang) {
+        cflags = [ "-Wno-incompatible-pointer-types" ]
+      }
+    }
+    configs += [ ":zlib_warnings" ]
   }
 
   configs -= [ "//build/config/compiler:chromium_code" ]

zlib.gyp

@@ -81,6 +81,11 @@
       'conditions': [
         ['OS!="ios" and (target_arch=="ia32" or target_arch=="x64")', {
           'sources' : [ 'x86.c', ],
+          'variables': {
+            'clang_warning_flags': [
+              '-Wno-incompatible-pointer-types',
+            ],
+          },
         }],
         ['OS!="win"', {
           'product_name': 'chrome_zlib',