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
In commit 3dab4682f6 (FindPython: reduces consumption of resources,
2020-02-10, v3.17.0-rc1~11^2) we accidentally broke the python
executable version validation when the "LOCATION" strategy is used
with the plain `FindPython` module. Fix the logic and add test
cases covering those combinations.
Fixes: #20465
