find_package: Search a "system package registry"
Generalize the "user package registry" created by commit ed0650f6 (Teach find_package to search a "package registry", 2009-09-01). Define a corresponding "system" registry key under HKEY_LOCAL_MACHINE. This gives package installers a place to create a registry value that points at the right location for find_package() to locate the package.
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