Fix build failure with ICX 2026 on Windows

[xaleryb]

Problem

ICX 2026 on Windows strictly enforces that C99 _Complex double cannot be passed to UCRT's creal()/cimag() functions, which expect _Dcomplex (an MSVC struct type). This causes hard compilation errors in all three build targets (mkl_umath_loops, _ufuncs, _patch_numpy).

Root cause chain:

1. CMakeLists.txt sets C_STANDARD 99 → CMake injects -Qstd:c99 for ICX
2. With -Qstd:c99, ICX enables strict C99 complex type semantics
3. numpy's npy_common.h defines npy_cdouble as double _Complex for __INTEL_LLVM_COMPILER (regardless of platform)
4. numpy's npy_math.h calls creal(z) / cimag(z) where z is npy_cdouble
5. UCRT's creal() expects _Dcomplex — ICX 2025 emitted a warning, ICX 2026 makes it a hard error

This is why Intel's own CI did not catch this: Intel's channel provides dpcpp_win-64 2025.x which was lenient. conda-forge uses dpcpp_win-64 2026.0 which enforces strict types.

Fix

Unset C_STANDARD for all three targets when building on Windows with IntelLLVM. Without -Qstd:c99, ICX operates in its default clang-cl compatibility mode and handles the _Complex/_Dcomplex coercion without error — matching the behavior of ICX 2025 and earlier.

if(WIN32 AND CMAKE_C_COMPILER_ID STREQUAL "IntelLLVM")
  set_target_properties(<target> PROPERTIES C_STANDARD "")
endif()

Note

The underlying incompatibility is in numpy's npy_math.h, which should guard creal/cimag calls for __INTEL_LLVM_COMPILER on Windows — similar to how npy_common.h already guards the npy_cdouble typedef. This fix works around it at the mkl_umath build level until numpy addresses it upstream.

[ndgrigorian]

@xaleryb
The meson-python build PR does not show this problem without the workaround:
https://github.com/IntelPython/mkl_umath/actions/runs/25949137641/job/76283388303?pr=166
and uses 2026.0 Windows compiler.

It does update how the loop generation works, though, so I'm not sure -- it's possible that this solution is still correct. We can try to build locally, or also, it would be good to drop the fix commit in this PR temporarily and see if CI passes.

If it does, it means there may be something specific to conda-forge or a recipe issue.