Cannot compile with LLVM/Ninja on Windows due to `/utf-8`

[aminya]

Fmt has started adding /utf-8 flags when installed through vcpkg on Windows even though it's not supported by Clang.
https://github.com/aminya/cpp_vcpkg_project/actions/runs/10810623863/job/29988266523?pr=55#step:6:213

Here's a full reproduction:
https://github.com/aminya/cpp_vcpkg_project/tree/a53c2ce04dd16a26a60a3c1d85d2df212977319a

Here are some generated code in build/vcpkg_installed/x64-windows/share/fmt/fmt-targets.cmake

# Create imported target fmt::fmt
add_library(fmt::fmt SHARED IMPORTED)

set_target_properties(fmt::fmt PROPERTIES
  INTERFACE_COMPILE_DEFINITIONS "FMT_SHARED"
  INTERFACE_COMPILE_FEATURES "cxx_std_11"
  INTERFACE_COMPILE_OPTIONS "\$<\$<COMPILE_LANGUAGE:CXX>:/utf-8>"
  INTERFACE_INCLUDE_DIRECTORIES "${_IMPORT_PREFIX}/include"
)

# Create imported target fmt::fmt-header-only
add_library(fmt::fmt-header-only INTERFACE IMPORTED)

set_target_properties(fmt::fmt-header-only PROPERTIES
  INTERFACE_COMPILE_DEFINITIONS "FMT_HEADER_ONLY=1"
  INTERFACE_COMPILE_FEATURES "cxx_std_11"
  INTERFACE_COMPILE_OPTIONS "\$<\$<COMPILE_LANGUAGE:CXX>:/utf-8>"
  INTERFACE_INCLUDE_DIRECTORIES "${_IMPORT_PREFIX}/include"
)

This should be conditional. Although vcpkg might use its own toolchain, it doesn't mean the cmake file is going to be used with MSVC. This includes clang-tidy usage (different at build time than configuration time).