Clang failing compiling itself when make buildworld

Finally I am trying to update from 11-RC1 to releng according to the guide here.
First problem occurred after I deleted the /usr/src.
svn failed because there was no IPv6 enabled in the kernel config.
So to make a kernel with IPv6 enabled I downloaded src.txz from 11 stable, unpacked it into /usr/src and uttered make buildworld KERNCONF=GENERIC.
Now this aborted while building llvm complaining about an ambiguous abs() call.
/usr/src/lib/clang/libllvmanalysis/../../../contrib/llvm/lib/Analysis/BasicAliasAnalysis.cpp:1564:52: error: call to 'abs' is ambiguous
  uint64_t MinDiffBytes = MinDiff.getZExtValue() * std::abs(Var0.Scale);
I guess I did some horribly wrong again.
Any idea how to fix that situation?

Edit: I am leaving the PC doing a make buildkernel only overnight. Maybe I am just confusing something. I better have a nap now...