[mlir][Python] Fix the last remaining instance of PYTHON_EXECUTABLE.

[stellaraccident]

Summary

• Was causing auto-detect of pybind11 to fail on clean configure.

mlir/CMakeLists.txt

 
 option(MLIR_INCLUDE_INTEGRATION_TESTS
        "Generate build targets for the MLIR integration tests.")
 
 #-------------------------------------------------------------------------------
 # Python Bindings Configuration
 # Requires:
 #   The pybind11 library can be found (set with -DPYBIND_DIR=...)
-#   The python executable is correct (set with -DPYTHON_EXECUTABLE=...)
+#   The python executable is correct (set with -DPython3_EXECUTABLE=...)
 #
 # Version locking
 # ---------------
 # By default, python extensions are version locked to specific Python libraries.
 # This linking mode is somewhat more consistent across platforms and surfaces
 # undefined symbols at link time (vs runtime). It is suitable for development
 # workflows but can be disabled for more flexible deployment by
 # setting -DMLIR_PYTHON_BINDINGS_VERSION_LOCKED=OFF

mlir/cmake/modules/MLIRDetectPythonEnv.cmake

 # Macros and functions related to detecting details of the Python environment.
 
 # Detects a pybind11 package installed in the current python environment
 # and sets variables to allow it to be found. This allows pybind11 to be
 # installed via pip, which typically yields a much more recent version than
 # the OS install, which will be available otherwise.
 function(mlir_detect_pybind11_install)
   if(pybind11_DIR)
     message(STATUS "Using explicit pybind11 cmake directory: ${pybind11_DIR} (-Dpybind11_DIR to change)")
   else()
     message(CHECK_START "Checking for pybind11 in python path...")
     execute_process(
-      COMMAND "${PYTHON_EXECUTABLE}"
+      COMMAND "${Python3_EXECUTABLE}"
       -c "import pybind11;print(pybind11.get_cmake_dir(), end='')"
       WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
       RESULT_VARIABLE STATUS
       OUTPUT_VARIABLE PACKAGE_DIR
       ERROR_QUIET)
     if(NOT STATUS EQUAL "0")
       message(CHECK_FAIL "not found (install via 'pip install pybind11' or set pybind11_DIR)")
       return()
     endif()
     message(CHECK_PASS "found (${PACKAGE_DIR})")
     set(pybind11_DIR "${PACKAGE_DIR}" PARENT_SCOPE)
   endif()
 endfunction()