I started writing the macro that chooses between ‘glibc/linux’ and ‘glibc/hurd’ so I added #:export (glibc) at defined modules, renamed glibc to glibc/linux and then I added
(define (glibc-for-target target)
"Return the glibc for TARGET, glibc/linux for a linux host or
glibc/hurd for a hurd host"
(match target
("i686-pc-gnu" glibc/hurd)
(_ glibc/linux)))
(define-syntax glibc
(identifier-syntax (glibc-for-target (%current-target-system))))