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BUG: 436155

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We want to know here if the standard attribute is available

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CMake >= 3.0 supports bracket comments, and the reStructuredText
integration code in sphinx/ext/ecm.py already supports extracting
the docs from a bracket comment instead.
Editing documentation without leading line comment markers is more simple,
e,g. when reflowing text over lines.

With ECM meanwhile requiring CMake 3.5 now it is possible to switch
(and thus follow also the approach used by cmake itself).

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That's the case when using KDE_L10N_AUTO_TRANSLATIONS, and the code handles
this case above when looking for the right po location already, but that
result was accidentally discarded here.

To prevent accidental use of the C++-side hex number-style version variants
on the cmake side where human-readable style is used instead (which can
happen due to the similar names of the cmake variables/arguments
and C++ macros which screws up developers' brain) we better do some
proper input checks also on the arguments EXCLUDE_DEPRECATED_BEFORE_AND_AT
and DEPRECATED_BASE_VERSION