[cmake-developers] Fix for FindPythonLibs with multiple configurations

[Richard Christie]

Dear CMake developers!

I have had to modify Module FindPythonLibs.cmake to correctly find the Python debug and release libraries on Windows (for Python 3.5).

A problem with the current version is that it always copies the current PYTHON_LIBRARY to PYTHON_LIBRARY_RELEASE:

set(PYTHON_LIBRARY_RELEASE "${PYTHON_LIBRARY}")

this means when find_package(PythonLibs) is called multiple times when both release and debug python libraries are present, it nests the full multi-configuration library list within a new multi-configuration library list, so after getting the expected multi-configuration result:

optimized;RELEASE_LIBRARY;debug;DEBUG_LIBRARY

you get after a second call:

optimised;optimized;RELEASE_LIBRARY;debug;DEBUG_LIBRARY;debug;DEBUG_LIBRARY

and so on.

The attached patch fixes this nesting issue. It is slightly more symmetric in its treatment of DEBUG and RELEASE, however it relies on a RELEASE library variable existing from the last run to avoid copying a list into the PYTHON_RELEASE_LIBRARY variable, and allows only a single library to be pre-specified. These issues are somewhat unsatisfactory to me and there may be a better idiom for dealing with pre-specified multi-configuration libraries.

Regards,
Richard

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