Consider using the MSVC linker option /WX if the AVIF_ENABLE_WERROR cmake option is ON

[wantehchang]

The MSVC linker has a /WX option to treat linker warnings as errors. It may make sense for libavif to use the /WX linker option if the AVIF_ENABLE_WERROR cmake option is ON.

My only concern is that AVIF_ENABLE_WERROR is about treating compiler warnings as errors, so we are using it for a different (but related) purpose.

[wantehchang]

I attached a prototype patch that adds the /WX linker option to avif.dll, avifenc.exe, and avifdec.exe: msvc-linker-WX-patch.txt