macOS: Only build OneDalNative when oneDAL headers & libs are configured (fixes macOS x64 build)

[asp2286]

macOS: Only build OneDalNative when oneDAL headers & libs are configured (fixes macOS x64 build)

Problem
On macOS x64, building src/Native fails with:

fatal error: 'daal.h' file not found

The oneDAL “devel” (headers) package is not available via NuGet for osx-x64, so locally ONEDAL_DEVEL_PATH is not present and OneDalNative cannot compile. Today the native CMake unconditionally includes OneDalNative on x64, which makes a default macOS developer build fail.

What this PR changes

• Gate add_subdirectory(OneDalNative) behind explicit configuration:
  • Only include OneDalNative when both ONEDAL_DEVEL_PATH and ONEDAL_REDIST_PATH are defined and ${ONEDAL_DEVEL_PATH}/include/daal.h exists.
  • Otherwise, print an informative message(STATUS ...) and skip OneDalNative.

Why this is safe

• Windows/Linux CI builds that pass ONEDAL_* continue to build OneDalNative unchanged.
• macOS builds (where oneDAL headers aren’t available via NuGet by default) no longer fail—they build the rest of the native components and skip OneDalNative.
• No changes to build.sh are required. The script already sets -DONEDAL_* only when callers pass the corresponding command-line switches.

Repro (before)

# macOS x64 without oneDAL headers
cmake ... && make
# -> OneDalNative/OneDalAlgorithms.cpp: fatal error: 'daal.h' file not found

Behavior (after)

• If ONEDAL_* not provided: CMake prints Skipping OneDalNative: ONEDAL_DEVEL_PATH/ONEDAL_REDIST_PATH not set and the rest builds fine.
• If ONEDAL_* provided and ${ONEDAL_DEVEL_PATH}/include/daal.h exists: OneDalNative builds as before.

Test matrix

• ✅ macOS x64 (Intel): builds successfully; OneDalNative skipped by default; other native targets build.
• ✅ macOS arm64 (Apple Silicon): previously didn’t include OneDalNative (arch guard); behavior unchanged.
• ✅ Linux/Windows (CI): when ONEDAL_* are set (as in existing pipelines), behavior unchanged and OneDalNative builds.

Notes

• This PR intentionally doesn’t alter OpenMP handling or SymSgdNative. On macOS x64, developers who need SymSgdNative can use Homebrew LLVM (brew install llvm libomp) or provide appropriate OpenMP flags; that is orthogonal to oneDAL headers availability.
• We can later add an explicit -DMLNET_BUILD_ONEDAL=ON/OFF flag for stricter control if maintainers prefer.

[asp2286]

Reviewer checklist (suggested)

Scope & Risk

• Change limited to src/Native/CMakeLists.txt only (no code changes).
• Behavior unchanged when ONEDAL_* are provided (Windows/Linux CI).
• macOS local builds no longer fail by default (OneDalNative skipped).

Build verification

• macOS x64 (no ONEDAL_* set): cmake && make succeeds; CMake prints STATUS "Skipping OneDalNative...".
• macOS arm64: behavior unchanged (OneDalNative was not built before).
• Windows/Linux with ONEDAL_* (CI or local): OneDalNative builds as before.

CMake output sanity

• STATUS message shows when skipping OneDalNative.
• When ONEDAL_* are provided and ${ONEDAL_DEVEL_PATH}/include/daal.h exists, OneDalNative target is present.

Documentation / Notes

• PR description explains context (NuGet lack of oneDAL headers on osx-x64) and rationale.
• Commit message is clear and references macOS build fix.