issue #11760 \doxyconfig does not work as an if-argument

[albert-github]

Adding the possibility to use the \doxyconfig command in the section label of \if

[doxygen]

@albert-github This makes it work for \if but not for \cond. I think it would be better to do the \doxyconfig handling inside the CondParser.

[albert-github]

That is indeed the case I created a "proof of concept" working for the \if command and thought about adding it somewhere else in a generic way as well. I thought I would have to change the constexpr parser for this (and was not fond of it to do that, also seen the limited time I had at that moment). Feel free to to create an alternative implementation.