[lldb/CMake] Make it possible to build against Python 2 with CMake > 3.12

[JDevlieghere]

Summary

In addition to having the default fallback from Python 3 to Python 2, it should also be possible to build against Python 2 explicitly. This patch makes that possible by setting LLDB_PYTHON_VERSION.

[compnerd]

Would be nice to remove this entirely in favour of CMake's builtin support for Python Interpeter and Libraries.

[JDevlieghere]

In D81501#2083359, @compnerd wrote:

Would be nice to remove this entirely in favour of CMake's builtin support for Python Interpeter and Libraries.

It'll continue to exist to find Python and SWIG atomically, but at least once we bump the minimum requirement for CMake to 3.12 this file will be a lot simpler.

lldb/cmake/modules/FindPythonInterpAndLibs.cmake

 #.rst:
 # FindPythonInterpAndLibs
 # -----------
 #
 # Find the python interpreter and libraries as a whole.
 
-if(PYTHON_LIBRARIES AND PYTHON_INCLUDE_DIRS AND PYTHON_EXECUTABLE AND SWIG_EXECUTABLE)
-  set(PYTHONINTERPANDLIBS_FOUND TRUE)
-else()
-  find_package(SWIG 2.0)
-  if (SWIG_FOUND)
-    if(NOT CMAKE_VERSION VERSION_LESS 3.12)
-      # Use PYTHON_HOME as a hint to find Python 3.
-      set(Python3_ROOT_DIR "${PYTHON_HOME}")
-      find_package(Python3 COMPONENTS Interpreter Development)
-      if(Python3_FOUND AND Python3_Interpreter_FOUND)
-        set(PYTHON_LIBRARIES ${Python3_LIBRARIES})
-        set(PYTHON_INCLUDE_DIRS ${Python3_INCLUDE_DIRS})
-        set(PYTHON_EXECUTABLE ${Python3_EXECUTABLE})
+macro(FindPython3)
 
-        # The install name for the Python 3 framework in Xcode is relative to
-        # the framework's location and not the dylib itself.
-        #
-        #   @rpath/Python3.framework/Versions/3.x/Python3
-        #
-        # This means that we need to compute the path to the Python3.framework
-        # and use that as the RPATH instead of the usual dylib's directory.
-        #
-        # The check below shouldn't match Homebrew's Python framework as it is
-        # called Python.framework instead of Python3.framework.
-        if (APPLE AND Python3_LIBRARIES MATCHES "Python3.framework")
-          string(FIND "${Python3_LIBRARIES}" "Python3.framework" python_framework_pos)
-          string(SUBSTRING "${Python3_LIBRARIES}" "0" ${python_framework_pos} PYTHON_RPATH)
-        endif()
 
-        mark_as_advanced(
-          PYTHON_LIBRARIES
-          PYTHON_INCLUDE_DIRS
-          PYTHON_EXECUTABLE
-          PYTHON_RPATH
-          SWIG_EXECUTABLE)
-      elseif(NOT CMAKE_SYSTEM_NAME STREQUAL Windows)
-        # Use PYTHON_HOME as a hint to find Python 2.
-        set(Python2_ROOT_DIR "${PYTHON_HOME}")
-        find_package(Python2 COMPONENTS Interpreter Development)
-        if(Python2_FOUND AND Python2_Interpreter_FOUND)
-          set(PYTHON_LIBRARIES ${Python2_LIBRARIES})
-          set(PYTHON_INCLUDE_DIRS ${Python2_INCLUDE_DIRS})
-          set(PYTHON_EXECUTABLE ${Python2_EXECUTABLE})
-          mark_as_advanced(
-            PYTHON_LIBRARIES
-            PYTHON_INCLUDE_DIRS
-            PYTHON_EXECUTABLE
-            SWIG_EXECUTABLE)
-        endif()
+    set(PYTHON3_FOUND TRUE)
+  endif()
+endmacro()
+
+macro(FindPython2)
+
+    set(PYTHON2_FOUND TRUE)
+endmacro()
+
+if(PYTHON_LIBRARIES AND PYTHON_INCLUDE_DIRS AND PYTHON_EXECUTABLE AND SWIG_EXECUTABLE)
+  set(PYTHONINTERPANDLIBS_FOUND TRUE)
+else()
+  find_package(SWIG 2.0)
+  if (SWIG_FOUND)
+    if(NOT CMAKE_VERSION VERSION_LESS 3.12)
+      if (LLDB_PYTHON_VERSION)
+        if (LLDB_PYTHON_VERSION VERSION_EQUAL "2")
+          FindPython2()
+        elseif(LLDB_PYTHON_VERSION VERSION_EQUAL "3")
+          FindPython3()
+        endif()
+      else()
+        FindPython3()
+        if (NOT PYTHON3_FOUND AND NOT CMAKE_SYSTEM_NAME STREQUAL Windows)
+          FindPython2()
+        endif()
       endif()
     else()
       find_package(PythonInterp)
       find_package(PythonLibs)
       if(PYTHONINTERP_FOUND AND PYTHONLIBS_FOUND AND SWIG_FOUND)
         if (NOT CMAKE_CROSSCOMPILING)
           string(REPLACE "." ";" pythonlibs_version_list ${PYTHONLIBS_VERSION_STRING})
           list(GET pythonlibs_version_list 0 pythonlibs_major)