[lldb/CMake] Only auto-enable Python when SWIG is found

As correctly pointed out by Martin on the mailing list, Python should
only be auto-enabled if SWIG is found as well. This moves the logic of
finding SWIG into FindPythonInterpAndLibs to make that possible.

To make diagnosing easier I've included a status message to convey why
Python support is disabled.

lldb/cmake/modules/FindPythonInterpAndLibs.cmake

@@ -4,40 +4,47 @@
 #
 # Find the python interpreter and libraries as a whole.
 
-if(PYTHON_LIBRARIES AND PYTHON_INCLUDE_DIRS AND PYTHON_EXECUTABLE)
+if(PYTHON_LIBRARIES AND PYTHON_INCLUDE_DIRS AND PYTHON_EXECUTABLE AND SWIG_EXECUTABLE)
   set(PYTHONINTERPANDLIBS_FOUND TRUE)
 else()
-  if ("${CMAKE_SYSTEM_NAME}" STREQUAL "Windows")
-    find_package(Python3 COMPONENTS Interpreter Development QUIET)
-    if (Python3_FOUND AND Python3_Interpreter_FOUND)
-      set(PYTHON_LIBRARIES ${Python3_LIBRARIES})
-      set(PYTHON_INCLUDE_DIRS ${Python3_INCLUDE_DIRS})
-      set(PYTHON_EXECUTABLE ${Python3_EXECUTABLE})
-      mark_as_advanced(
-        PYTHON_LIBRARIES
-        PYTHON_INCLUDE_DIRS
-        PYTHON_EXECUTABLE)
-    endif()
-  else()
-    find_package(PythonInterp QUIET)
-    find_package(PythonLibs QUIET)
-    if(PYTHONINTERP_FOUND AND PYTHONLIBS_FOUND)
-      if (NOT CMAKE_CROSSCOMPILING)
-        string(REPLACE "." ";" pythonlibs_version_list ${PYTHONLIBS_VERSION_STRING})
-        list(GET pythonlibs_version_list 0 pythonlibs_major)
-        list(GET pythonlibs_version_list 1 pythonlibs_minor)
+  find_package(SWIG 2.0 QUIET)
+  if (SWIG_FOUND)
+    if ("${CMAKE_SYSTEM_NAME}" STREQUAL "Windows")
+      find_package(Python3 COMPONENTS Interpreter Development QUIET)
+      if (Python3_FOUND AND Python3_Interpreter_FOUND)
+        set(PYTHON_LIBRARIES ${Python3_LIBRARIES})
+        set(PYTHON_INCLUDE_DIRS ${Python3_INCLUDE_DIRS})
+        set(PYTHON_EXECUTABLE ${Python3_EXECUTABLE})
+        mark_as_advanced(
+          PYTHON_LIBRARIES
+          PYTHON_INCLUDE_DIRS
+          PYTHON_EXECUTABLE
+          SWIG_EXECUTABLE)
+      endif()
+    else()
+      find_package(PythonInterp QUIET)
+      find_package(PythonLibs QUIET)
+      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)
+          list(GET pythonlibs_version_list 1 pythonlibs_minor)
 
-        # Ignore the patch version. Some versions of macOS report a different
-        # patch version for the system provided interpreter and libraries.
-        if (CMAKE_CROSSCOMPILING OR (PYTHON_VERSION_MAJOR VERSION_EQUAL pythonlibs_major AND
-            PYTHON_VERSION_MINOR VERSION_EQUAL pythonlibs_minor))
-          mark_as_advanced(
-            PYTHON_LIBRARIES
-            PYTHON_INCLUDE_DIRS
-            PYTHON_EXECUTABLE)
+          # Ignore the patch version. Some versions of macOS report a different
+          # patch version for the system provided interpreter and libraries.
+          if (CMAKE_CROSSCOMPILING OR (PYTHON_VERSION_MAJOR VERSION_EQUAL pythonlibs_major AND
+              PYTHON_VERSION_MINOR VERSION_EQUAL pythonlibs_minor))
+            mark_as_advanced(
+              PYTHON_LIBRARIES
+              PYTHON_INCLUDE_DIRS
+              PYTHON_EXECUTABLE
+              SWIG_EXECUTABLE)
+          endif()
         endif()
       endif()
     endif()
+  else()
+    message(STATUS "SWIG 2 or later is required for Python support in LLDB but could not be found")
   endif()
 
   include(FindPackageHandleStandardArgs)
@@ -47,5 +54,6 @@ else()
                                     REQUIRED_VARS
                                       PYTHON_LIBRARIES
                                       PYTHON_INCLUDE_DIRS
-                                      PYTHON_EXECUTABLE)
+                                      PYTHON_EXECUTABLE
+                                      SWIG_EXECUTABLE)
 endif()

lldb/scripts/CMakeLists.txt

@@ -15,12 +15,6 @@ if(LLDB_BUILD_FRAMEWORK)
   set(framework_arg --framework --target-platform Darwin)
 endif()
 
-find_package(SWIG REQUIRED)
-set(SWIG_MIN_VERSION "2.0.0")
-if (${SWIG_VERSION} VERSION_LESS ${SWIG_MIN_VERSION})
-  message(FATAL_ERROR "LLDB requires swig ${SWIG_MIN_VERSION}, your version is ${SWIG_VERSION}.")
-endif()
-
 if(APPLE)
   set(DARWIN_EXTRAS "-D__APPLE__")
 else()
@@ -38,7 +32,6 @@ set(SWIG_COMMON_FLAGS
   -outdir ${CMAKE_CURRENT_BINARY_DIR}
 )
 
-
 if (LLDB_ENABLE_PYTHON)
   add_custom_command(
     OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/LLDBWrapPython.cpp