Build-script cannot find Blocks.h while building CF/Foundation?

I'm trying to reproduce a buildbot error with ./swift/utils/build-script and ./swift/utils/update-checkout --clone, but I'm finding that CoreFoundation/Foundation cannot build due to the following error:

../build/buildbot_incremental/foundation-linux-x86_64/Foundation/usr//lib/swift/CoreFoundation/CFInternal.h:96:10: fatal error: 'Block.h' file not found
#include <Block.h>
         ^~~~~~~~~
1 error generated.

Has anybody seen this before? If it matters, I'm building on Red Hat Fedora 28.

I don't want to waste your time, but did you remember to install libblocksruntime-dev on this machine? That's the error message I've seen other people hit when they forget that step.

That solved it (named libblocksruntime and libblocksruntime-devel on Fedora).

For various reasons, I assumed that swift would build the blocks runtime if it needed it. I also assumed that Fedora wouldn't have a "blocks runtime" package.