Why are BOOL cache variables sometimes used instead of options?

In the main CMake file some configurational variables are set with `option()` while some are set as BOOL cache variables (`G2O_BUILD_APPS` and `G2O_BUILD_EXAMPLES` for example). Is there a reason for this?



I am asking because even these ways of setting variables have similar functionality, there are subtle differences in the behavior of `set(CACHE)` and `option()` commands. The one that is important to me is that when `option()` is used, it can be overridden with a non-cache variable (see [CMP0077](https://cmake.org/cmake/help/latest/policy/CMP0077.html)), while `set(CACHE)` can only be overridden with cache variables (seems like it is true until CMake 3.22, see [CMP0126](https://cmake.org/cmake/help/latest/policy/CMP0126.html#policy:CMP0126)) which are visible to users until `INTERNAL` is used.



This is not a big deal, just the resulting code becomes less compact. If there is no particular reason, I would suggest using `option()`.

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