[fmt] Add a way to disable FMT_UNICODE when building fmt

Is your feature request related to a problem? Please describe.

Hi, I have a small test project built on Windows with MSVC, with fmt 11.0.2#1 installed through vcpkg.

My test project uses the /source-charset:windows-1252 compile option for its own sources:

cmake_minimum_required(VERSION 3.29.0 FATAL_ERROR)

set(CMAKE_TOOLCHAIN_FILE "${VCPKG_ROOT}/scripts/buildsystems/vcpkg.cmake" CACHE PATH "")

project(Test CXX)

find_package(fmt CONFIG REQUIRED)

add_executable(main
    main.cpp
)

target_compile_options(main PRIVATE
    /source-charset:windows-1252
)

target_link_libraries(main PRIVATE
    fmt::fmt
)

However, when try to build it I get the following error:

C:\PROGRA~1\MIB055~1\2022\PROFES~1\VC\Tools\MSVC\1441~1.341\bin\Hostx64\x64\cl.exe  /nologo /TP -DFMT_SHARED -external:IC:\Users\martin\fmt_test\build\vcpkg_installed\x64-windows\include -external:W0 /DWIN32 /D_WINDOWS /EHsc /Ob0 /Od /RTC1 -MDd -Zi /source-charset:windows-1252 /utf-8 /showIncludes /FoCMakeFiles\main.dir\main.cpp.obj /FdCMakeFiles\main.dir\ /FS -c C:\Users\martin\fmt_test\main.cpp
cl : Command line error D8016 : '/source-charset:windows-1252' and '/utf-8' command-line options are incompatible

I believe the reason for this is the /utf-8 option being set as PUBLIC for the fmt target, here:

https://github.com/fmtlib/fmt/blob/0c9fce2ffefecfdce794e1859584e25877b7b592/CMakeLists.txt#L362

which results on the following code being generated by vcpkg's CMake machinery (on fmt-targets.cmake):

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

I believe this ends up propagating the /utf-8 option to my own sources, and thus breaking the build.

Proposed solution

I reported this to fmt (see https://github.com/fmtlib/fmt/issues/4221) and the maintainer indicated that /utf-8 can be disabled through FMT_UNICODE, and it's an issue on the vcpkg side. Perhaps there could be done with a port feature?