fix: pass /utf-8 only if the compiler is MSVC at build time

[aminya]

Fixes #4158

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 will be used with MSVC for all the users of fmt. This includes clang-tidy usage (different at build time than configuration time).

[vitaut]

Thanks for the fix!

[aminya]

@vitaut you're welcome. Is there a way I can backport this to a previous version as a hotfix? It's blocking fmt's usage through vcpkg on Windows with LLVM.
Or when is the expected date for the next release?

[aminya]

For now, I am adding a patch to vcpkg
microsoft/vcpkg#40944

[vitaut]

We could make a new release but since you've already submitted a patch for vcpkg let's just go with it.