Fix build failure with ICX 2026 on Windows

ICX 2026 in clang-cl mode 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 when C_STANDARD 99 is set, because CMake translates that
to -Qstd:c99 for ICX, enabling strict C99 complex type semantics.

numpy's npy_common.h defines npy_cdouble as 'double _Complex' for
IntelLLVM compilers regardless of platform. numpy's npy_math.h then
calls creal()/cimag() on npy_cdouble values. Under ICX 2025 this was
a warning; under ICX 2026 it is a hard error.

Fix: on Windows with IntelLLVM, unset C_STANDARD for all three targets
so CMake does not inject -Qstd:c99. ICX in its default clang-cl
compatibility mode handles the _Complex/_Dcomplex coercion without
error, matching the behavior of ICX 2025 and earlier.

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.

Author: xaleryb

CMakeLists.txt

@@ -109,6 +109,14 @@ set_target_properties(${_trgt} PROPERTIES
     CMAKE_POSITION_INDEPENDENT_CODE ON
     C_STANDARD 99
 )
+if(WIN32 AND CMAKE_C_COMPILER_ID STREQUAL "IntelLLVM")
+  # ICX 2026 on Windows strictly enforces that C99 _Complex double (npy_cdouble as defined
+  # by numpy for IntelLLVM) cannot be passed to UCRT's creal/cimag which expect _Dcomplex.
+  # This is triggered by the -Qstd:c99 flag injected via C_STANDARD 99 above.
+  # Removing the C standard constraint for ICX on Windows leaves the compiler in its
+  # default clang-cl compatibility mode, which does not enforce this type mismatch.
+  set_property(TARGET ${_trgt} PROPERTY C_STANDARD)
+endif()
 target_include_directories(${_trgt} PUBLIC mkl_umath/src/ ${Python_NumPy_INCLUDE_DIRS} ${Python_INCLUDE_DIRS})
 target_link_libraries(${_trgt} PUBLIC MKL::MKL ${Python_LIBRARIES})
 target_link_options(${_trgt} PUBLIC ${_linker_options})
@@ -129,6 +137,9 @@ target_compile_definitions(_ufuncs PUBLIC NPY_NO_DEPRECATED_API=NPY_1_7_API_VERS
 target_link_options(_ufuncs PRIVATE ${_linker_options})
 target_link_libraries(_ufuncs PRIVATE mkl_umath_loops)
 set_target_properties(_ufuncs PROPERTIES C_STANDARD 99)
+if(WIN32 AND CMAKE_C_COMPILER_ID STREQUAL "IntelLLVM")
+  set_property(TARGET _ufuncs PROPERTY C_STANDARD)
+endif()
 if (UNIX)
   set_target_properties(_ufuncs PROPERTIES INSTALL_RPATH "$ORIGIN/../..;$ORIGIN/../../..;$ORIGIN")
 endif()
@@ -140,6 +151,9 @@ target_include_directories(_patch_numpy PRIVATE "mkl_umath/src/" ${Python_NumPy_
 target_compile_definitions(_patch_numpy PUBLIC NPY_NO_DEPRECATED_API=NPY_1_7_API_VERSION)
 target_link_libraries(_patch_numpy PRIVATE mkl_umath_loops)
 set_target_properties(_patch_numpy PROPERTIES C_STANDARD 99)
+if(WIN32 AND CMAKE_C_COMPILER_ID STREQUAL "IntelLLVM")
+  set_property(TARGET _patch_numpy PROPERTY C_STANDARD)
+endif()
 if (UNIX)
   set_target_properties(_patch_numpy PROPERTIES INSTALL_RPATH "$ORIGIN/../..;$ORIGIN/../../..;$ORIGIN")
 endif()