When compiling C++ files, Zig unconditionally passes -nostdinc++ and,
if link_libcpp is set, adds its bundled libc++/libc++abi include paths
as replacements (see Compilation.zig). On MSVC targets this conflicts
with the MSVC C++ runtime headers (vcruntime_typeinfo.h,
vcruntime_exception.h, etc.), causing compilation failures in SIMD
C++ code.
The fix is to use linkLibC instead of linkLibCpp on MSVC. Zig always
passes -nostdinc to strip default search paths, but LibCDirs.detect
re-adds the MSVC SDK include directories, which contain both C and
C++ standard library headers. This gives us proper access to MSVC's
own <optional>, <iterator>, <cstddef>, etc. without the libc++
conflicts.
For the package builds (highway, simdutf, utfcpp) this means
switching from linkLibCpp to linkLibC on MSVC. For SharedDeps and
GhosttyZig, linkLibC is already called separately, so we just skip
linkLibCpp.
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