Let user know when CMAKE_CXX_FLAGS are being ignored.

[stefanatwork]

Fixes #513

[Copilot]

Pull request overview

This PR addresses #513 by surfacing a CMake warning when users set CMAKE_CXX_FLAGS but Embree is configured to ignore them via EMBREE_IGNORE_CMAKE_CXX_FLAGS=ON, helping prevent confusing “flags not applied” build behavior.

Changes:

• Add a MESSAGE(WARNING ...) when CMAKE_CXX_FLAGS is non-empty and EMBREE_IGNORE_CMAKE_CXX_FLAGS is enabled.
• Apply the warning consistently across the Intel, GNU, Clang, and DPCPP toolchain CMake modules.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
common/cmake/intel.cmake	Warn when CMAKE_CXX_FLAGS is about to be cleared due to EMBREE_IGNORE_CMAKE_CXX_FLAGS.
common/cmake/gnu.cmake	Same warning for GNU toolchain path before clearing CMAKE_CXX_FLAGS.
common/cmake/dpcpp.cmake	Same warning for DPCPP/non-Windows path before clearing CMAKE_CXX_FLAGS.
common/cmake/clang.cmake	Same warning for Clang/non-Windows path before clearing CMAKE_CXX_FLAGS.

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