build: use `find_package(Python3)` if available (llvm runtimes).

[vvereschaka]

Summary

This is primarily motivated by the desire to move from Python2 to
Python3. PYTHON_EXECUTABLE is ambiguous. This explicitly identifies
the python interpreter in use. Since the LLVM build seems to be able to
completed successfully with python3, use that across the build. The old
path aliases PYTHON_EXECUTABLE to be treated as Python3.

Based on commit cd84bfb8142bc7ff3a07a188ffb809f1d86d1fd7

[vvereschaka]

Hi @compnerd,
here is one more place, which needs a proper detection for Python3/Python2.
I just copied a part of your changes accordingly.

[compnerd]

llvm/runtimes/CMakeLists.txt

118

This was not fatal previously was it? Why make it fatal now?

139

Similar

[vvereschaka]

llvm/runtimes/CMakeLists.txt

118

It was fatal in commit https://github.com/llvm/llvm-project/commit/c4c3883b00d3a6aa657a5e3e515c90c9ea1f81c6
I just did copy/paste into a proper location.

[compnerd]

llvm/runtimes/CMakeLists.txt

118

Sure, because those projects used it with a failure being fatal. Here, the check is being performed without it being fatal. Note that there was no check in the CMakeLists.txt which checked if the python interpreter was found. If that exists, we should remove the old check and keep the check here only.

llvm/runtimes/CMakeLists.txt

   # Handle common options used by all runtimes.
   include(AddLLVM)
   include(HandleLLVMOptions)
-  include(FindPythonInterp)
+
+  if(CMAKE_VERSION VERSION_LESS 3.12)
+    include(FindPythonInterp)
+    if( NOT PYTHONINTERP_FOUND )
+      message(FATAL_ERROR
+    "Unable to find Python interpreter, required for builds and testing.
+
+    Please install Python or specify the PYTHON_EXECUTABLE CMake variable.")
+    endif()
+
+    if( ${PYTHON_VERSION_STRING} VERSION_LESS 2.7 )
+      message(FATAL_ERROR "Python 2.7 or newer is required")
+    endif()
+
+    add_executable(Python3::Interpreter IMPORTED)
+    set_target_properties(Python3::Interpreter PROPERTIES
+      IMPORTED_LOCATION ${PYTHON_EXECUTABLE})
+    set(Python3_EXECUTABLE ${PYTHON_EXECUTABLE})
+  else()
+    find_package(Python3 COMPONENTS Interpreter)
+    if(NOT Python3_Interpreter_FOUND)
+      message(WARNING "Python3 not found, using python2 as a fallback")
+      find_package(Python2 COMPONENTS Interpreter REQUIRED)
+      if(Python2_VERSION VERSION_LESS 2.7)
+        message(SEND_ERROR "Python 2.7 or newer is required")
+      endif()
+
+      # Treat python2 as python3
+      add_executable(Python3::Interpreter IMPORTED)
+      set_target_properties(Python3::Interpreter PROPERTIES
+        IMPORTED_LOCATION ${Python2_EXECUTABLE})
+      set(Python3_EXECUTABLE ${Python2_EXECUTABLE})
+    endif()
+  endif()
 
   # Remove the -nostdlib++ option we've added earlier.
   string(REPLACE "-nostdlib++" "" CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS}")