IPO/LTO support for ICX (IntelLLVM) compiler (#4402)

* IPO/LTO support for ICX (IntelLLVM) compiler

https://github.com/pybind/pybind11/issues/4080



* style: pre-commit fixes

* Add WARNING/HELP WANTED comment.
Co-authored-by: Martin Blanchard <martin.blanchard@siemens.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Ralf W. Grosse-Kunstleve <rwgk@google.com>

tools/pybind11Common.cmake

@@ -311,6 +311,16 @@ function(_pybind11_generate_lto target prefer_thin_lto)
         HAS_FLTO "-flto${cxx_append}" "-flto${linker_append}" PYBIND11_LTO_CXX_FLAGS
         PYBIND11_LTO_LINKER_FLAGS)
     endif()
+  elseif(CMAKE_CXX_COMPILER_ID MATCHES "IntelLLVM")
+    # IntelLLVM equivalent to LTO is called IPO; also IntelLLVM is WIN32/UNIX
+    # WARNING/HELP WANTED: This block of code is currently not covered by pybind11 GitHub Actions!
+    if(WIN32)
+      _pybind11_return_if_cxx_and_linker_flags_work(
+        HAS_INTEL_IPO "-Qipo" "-Qipo" PYBIND11_LTO_CXX_FLAGS PYBIND11_LTO_LINKER_FLAGS)
+    else()
+      _pybind11_return_if_cxx_and_linker_flags_work(
+        HAS_INTEL_IPO "-ipo" "-ipo" PYBIND11_LTO_CXX_FLAGS PYBIND11_LTO_LINKER_FLAGS)
+    endif()
   elseif(CMAKE_CXX_COMPILER_ID MATCHES "Intel")
     # Intel equivalent to LTO is called IPO
     _pybind11_return_if_cxx_and_linker_flags_work(HAS_INTEL_IPO "-ipo" "-ipo"