Add the following variables:
* Python_EXECUTABLE_DEBUG
* Python_INTERPRETER
* Python_DEBUG_POSTFIX
and target Python::InterpreterDebug.
python_add_library() command Manage DEBUG_POSTFIX target property based
on the value of Python_DEBUG_POSTFIX variable.
Fixes: #25874
Through hint Python_FIND_IMPLEMENTATIONS it is possble to specify, as an
ordered list, which implementations must be searched for.
Currently possible values are:
* CPython
* IronPython
