This marks all `<PACKAGENAME>_FOUND` result variables as deprecated
where possible (for `<PackageName>` find modules) to make it clearer
which variable to use.
In CMake 3.3, the FindPackageHandleStandardArgs module was refactored to
set both `<PackageName>_FOUND` and uppercase `<PACKAGENAME>_FOUND`
result variables to the same values. Before that, the FOUND_VAR argument
could be used to set the result variable.
* FindMatlab: Uppercased MATLAB_FOUND is not mentioned as it was never
documented.
* Documentation for FindPythonInterp and FindPythonLibs modules synced
accordingly to their deprecation (3.12 instead of 4.2).
* OPENGL_FOUND: deprecation version synced with other find modules.
* DevIL_FOUND was introduced in CMake 3.8. The uppercased variant not
mentioned as it was previously never documented.
Fixes: #27242
The `find_package_handle_standard_args(FOUND_VAR)` is deprecated as of
CMake 3.3 and both the `<PackageName>_FOUND` and the upper-cased
`<PACKAGE_NAME>_FOUND` are set with or without using this option.
Additionally, find modules and docs are also synced with this to make it
clearer.
Add it to a private source directory that is not installed so that we
can use it for building CMake itself. This will allow it to mature
before being distributed publicly.
