[mlir] Update Python CMake version requirement.

[mikeurbach]

Summary

The minimum required version is now 3.19 due to the usage of some
more recent features. Update the version check and error message
accordingly. Also remove some logic that behaved differently before
3.18, since we can assume we are now on version 3.19+.

[mikeurbach]

mlir/cmake/modules/MLIRDetectPythonEnv.cmake

21

It seemed useful to keep this comment, even though we are always on 3.18 after this patch.

[stella.stamenova]

LGTM. This makes https://reviews.llvm.org/D129163 obsolete

mlir/cmake/modules/MLIRDetectPythonEnv.cmake

 # Macros and functions related to detecting details of the Python environment.
 
 # Finds and configures python packages needed to build MLIR Python bindings.
 macro(mlir_configure_python_dev_packages)
-  if(CMAKE_VERSION VERSION_LESS "3.15.0")
+  if(CMAKE_VERSION VERSION_LESS "3.19.0")
   message(SEND_ERROR
       "Building MLIR Python bindings is known to rely on CMake features "
-      "that require at least version 3.15. Recommend upgrading to 3.18+ "
+      "that require at least version 3.19. Recommend upgrading to 3.19+ "
       "for full support. Detected current version: ${CMAKE_VERSION}")
   endif()
 
-  # After CMake 3.18, we are able to limit the scope of the search to just
-  # Development.Module. Searching for Development will fail in situations where
-  # the Python libraries are not available. When possible, limit to just
-  # Development.Module.
-  # See https://pybind11.readthedocs.io/en/stable/compiling.html#findpython-mode
-  if(CMAKE_VERSION VERSION_LESS "3.18.0")
-    message(WARNING
-        "This version of CMake is not compatible with statically built Python "
-        "installations. If Python fails to detect below this may apply to you. "
-        "Recommend upgrading to at least CMake 3.18. "
-        "Detected current version: ${CMAKE_VERSION}"
-    )
-    set(_python_development_component Development)
-  else()
-    if(MLIR_DETECT_PYTHON_ENV_PRIME_SEARCH)
-      # Prime the search for python to see if there is a full development
-      # package. This seems to work around cmake bugs searching only for
-      # Development.Module in some environments. However, in other environments
-      # it may interfere with the subsequent search for Development.Module.
-      find_package(Python3 ${LLVM_MINIMUM_PYTHON_VERSION}
-        COMPONENTS Interpreter Development)
-    endif()
-    set(_python_development_component Development.Module)
-  endif()
+
+  # After CMake 3.18, we are able to limit the scope of the search to just
+  # Development.Module. Searching for Development will fail in situations where
+  # the Python libraries are not available. When possible, limit to just
+  # Development.Module.
+  # See https://pybind11.readthedocs.io/en/stable/compiling.html#findpython-mode
+
   find_package(Python3 ${LLVM_MINIMUM_PYTHON_VERSION}
     COMPONENTS Interpreter ${_python_development_component} NumPy REQUIRED)
   unset(_python_development_component)
   message(STATUS "Found python include dirs: ${Python3_INCLUDE_DIRS}")
   message(STATUS "Found python libraries: ${Python3_LIBRARIES}")
   message(STATUS "Found numpy v${Python3_NumPy_VERSION}: ${Python3_NumPy_INCLUDE_DIRS}")
   mlir_detect_pybind11_install()
   find_package(pybind11 2.8 CONFIG REQUIRED)