Can you please include your build options (cmake command line) and attach build log (make --ouput-sync=target please to make it readable)? Perhaps some of the warnings need to be fixed. I'm assuming gcc-6 worked ok?
I spent last two weeks trying to get on what's going on. I did that on a 32-bit ARM hardware, which was itself slow and every rebuild took ages ...
However yesterday, just as I was collecting all the data you were asking for ... it magicly strated to work.
Overnight, I managed to completely test it and deliver MariaDB build to the stable.
Sooo ... Closed, I guess? 
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INFO:
- Yes, before that we used GCC 6.2
- Unfortunatelly I didn't catch the package set difference, which resolved the problem.
- In case you never tested it, now you know, MariaDB builds with GCC7
Thanks for getting back to us. Glad its working. Magic working isn't always good for us but at least its good for you. If something comes up again your story is here to give it credibility.
Happy MariaDBing in the mean time.
INFO: (in case somebody would use it in the future)
The same issue appeared during the same period for MySQL.
The issue were most likely solved by update of package 'gcc-c++' from 7.0.1-0.9.fc26 to 7.0.1-0.10.fc26.