Prepare for deprecating FindPythonInterp module. (#3040)

* Prepare for deprecating FindPythonInterp module.

Since FindPythonInterp module is deprecated after CMake 3.12, it is better to start using the new FindPython module.

* Add the proper punctuation in comments.

doc/CMakeLists.txt

@@ -6,7 +6,14 @@ if (NOT DOXYGEN)
   return ()
 endif ()
 
-find_package(PythonInterp QUIET REQUIRED)
+# Find the Python interpreter and set the PYTHON_EXECUTABLE variable.
+if (CMAKE_VERSION VERSION_LESS 3.12)
+  # This logic is deprecated in CMake after 3.12.
+  find_package(PythonInterp QUIET REQUIRED)
+else ()
+  find_package(Python QUIET REQUIRED)
+  set(PYTHON_EXECUTABLE ${Python_EXECUTABLE})
+endif ()
 
 add_custom_target(doc
   COMMAND ${PYTHON_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/build.py