Patch attached.

Unfortunately, I could not test it yet, since guix-package starts by 
building path(s) `/nix/store/9syafc8xvscaywjldyszzgkc8pljwpj6-gcc-cross-
boot0-4.7.2'

although gcc-4.7.2 (without "coss-boot") is already there. How come?
The previous mpc release built without problem without re-downloading gcc.

Actually, the same holds for the unmodified mpfr - after changing the
definition of mpc in multiprecision.scm, installing mpfr also starts by
redownloading gcc.

Is it related to the dependency of gcc on mpfr and mpc?

Andreas