libffi cannot be installed in user profile in core-updates

[Mark H Weaver <mhw@netris.org>]

On the Loongson 3A machine, I finished building all the packages from
core-updates that I use in my profile, and then tried installing them.
I got this:

--8<---------------cut here---------------start------------->8---
In /gnu/store/9sqx55rgygyrcyg350cfqwy8wqb4fr69-module-import/guix/build/union.scm:
 226: 1 [loop # #]
In unknown file:
   ?: 0 [mkdir "/gnu/store/c30bsax58scw5qwcnbma16k6z88s1lh3-profile/./include" ...]

ERROR: In procedure mkdir:
ERROR: In procedure mkdir: File exists
builder for `/gnu/store/17w6kggffh79drmmrdnxy1yhgr6p2in5-profile.drv' failed with exit code 1
--8<---------------cut here---------------end--------------->8---

The problem turns out to be that 'libffi' makes 'include' a symlink to
'lib/libffi-3.0.13/include', and union.scm cannot cope with conflicts
between a directory and a symlink.  Therefore, 'libffi' cannot be
installed in any user profile in core-updates (assuming that there's
some other installed package where 'include' is a directory).

This problem was introduced in d43547f12d4eae19845f3bf13a53a050af965063
"gnu: libffi: Leave a copy of headers where libffi.pc expects them."

     Mark