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