Comment # 1 on bug 1226445 from Stefan Dirsch

As usual Mesa was still building against Leap 15.5/15.6, but -drivers not. I've
now wiped all Mesa RPMs from the 15.5/`5.6 repositories. You need to go back to
15.6 Mesa/Mesa-drivers packages. Sorry. 

I should really give up on trying to get packages in X11:XOrg building against
Leap. It's just not doable.