[Martell0x1]

Summary

Fix propagation of /Zc:__cplusplus from spdlog::spdlog_header_only to CUDA compilation when using MSVC as the host compiler.

Problem

The header-only target currently propagates /Zc:__cplusplus unconditionally within the MSVC configuration. When a consumer target includes CUDA source files, this option can reach nvcc without being forwarded to the host compiler, resulting in a malformed compilation command.

Changes

• Restrict /Zc:__cplusplus to C++ compilation.
• Forward the option to the MSVC host compiler for CUDA compilation using -Xcompiler.

Validation

• Linux build succeeds.
• Existing tests pass.
• Windows/MSVC + CUDA compilation is verified, or the lack of access to this environment is documented.

Fixes #3425.

[gabime]

Thanks @Martell0x1