unknown type name '__BEGIN_DECLS' while building openmp offloading library for libcxxabi

Here's the command I got so far,

`
cd llvm-project-21.1.8.src
mkdir build
cd build
cmake ../llvm -G "Unix Makefiles" \
-C ../offload/cmake/caches/Offload.cmake \
-DCMAKE_BUILD_TYPE=Release \
-DCMAKE_INSTALL_PREFIX=

make
make install
`

But I'm facing some compilation errors,
`llvm-project-21.1.8.src/libcxxabi/src/cxa_guard_impl.h:57:
/usr/include/unistd.h:27:1: error: unknown type name '__BEGIN_DECLS'
27 | __BEGIN_DECLS`

Any idea how to fix the problem ?
I'm also wondering if the compiler can support openmp offloading to all GPUs from NVIDIA, AMD, and Intel?
Thanks,

Start by reproducing the documented CMake and Make commands from the issue, then inspect libcxxabi/src/cxa_guard_impl.h and the included /usr/include/unistd.h around the __BEGIN_DECLS diagnostic. Determine whether the failure is specific to this offloading configuration and document the supported GPU targets relevant to the question.

Written by the indexing model from the issue text.