I took the source rpm of mysql-5.1.36-6.7.2 (this is distributed for 11.2) and rebuilt it with slightly different parameters for configure. Nothing else changed except omitting one unnecessary patch and disabling innodb.
Result: it builds perfect for repo 11.2. I enabled a build for 11.1 as well, because I want the same version of mysql on different machines doing circular replication. It builds for 11.1 i586 but 11.1 x86_64 is marked as n/a. What can be the reason?
Unfortunately by now (I built last thursday and watched this morning for the last time when n/a was still current) the error has gone by itself. Not the slightest intervention from my side. This will be impossible to debug now. (scratching my head, where there is no hair left anyway).