[cmake-developers] Fix for FindPythonLibs with multiple configurations

[Brad King]

Hi Richard,

> The attached patch fixes this nesting issue.

Thanks for working on this.  Please look at revising this to avoid
exposing the _ANYCONFIG variable.  Use a "_Python_..." internal
variable that is unset, or find another way to do it without the
variable, such as `(PYTHON_RELEASE_LIBRARY OR PYTHON_DEBUG_LIBRARY)`.

> idiom for dealing with pre-specified multi-configuration libraries

I don't think our find modules ever consider the possibility of
pre-specified multi-config libraries.  Each cache entry is meant
to hold a single find_library result and the module puts them
together.  The leaking across multiple calls was not intentional.
Therefore this is not a concern for me.

Thanks,
-Brad