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FetchContent: Don't leak temporary variable and restore var robustly

Craig Scott requested to merge craig.scott/cmake:fc-MakeAvailable-leak-var into master

In FetchContent_MakeAvailable(), the __fcprefix__ previously used when pushing the value of CMAKE_EXPORT_FIND_PACKAGE_NAME to the variable stack is not needed. The stack will never be empty at that point, so pushing an empty value will be handled correctly. By removing the __fcprefix__, we no longer need any temporary variable when restoring CMAKE_EXPORT_FIND_PACKAGE_NAME.

But we need to ensure CMAKE_EXPORT_FIND_PACKAGE_NAME is left undefined if it wasn't defined before, and pushing an empty value doesn't let us distinguish between unset and set-but-empty. Therefore, when CMAKE_EXPORT_FIND_PACKAGE_NAME is undefined, push a specific value that can't be used by the project instead and check for that when popping it again. This ensures we can robustly distinguish the two cases and will always restore the right state.

Fixes: #25758 (closed)
Backport: release

Edited by Craig Scott

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