This patch adds the glibc headers for hurd. It assumes that glibc/hurd exists.

Ludovic if I try to use

>      (zero? (system* "make"
>                      (string-append out "/include/gnu/stubs.h")))))

as you suggested, I am getting

>make[2]: *** No rule to make target '/tmp/nix-build-glibc-hurd-headers-cross-i686-pc-gnu-2.18.drv-9/build/elf/soinit.os', needed by '/tmp/nix-build-glibc-hurd-headers-cross-i686-pc-gnu-2.18.drv-9/build/libc.so'.  Stop.
>make[2]: Leaving directory '/tmp/nix-build-glibc-hurd-headers-cross-i686-pc-gnu-2.18.drv-9/glibc-hurd-2.18/mach'
>Makefile:233: recipe for target 'mach/subdir_install' failed
>make[1]: *** [mach/subdir_install] Error 2
>make[1]: Leaving directory '/tmp/nix-build-glibc-hurd-headers-cross-i686-pc-gnu-2.18.drv-9/glibc-hurd-2.18'
>Makefile:9: recipe for target '/gnu/store/dgf45yi11yn08fq8hm1cp49yqhyg0nl6-glibc-hurd-headers-cross-i686-pc-gnu-2.18/include/gnu/stubs.h' failed
>make: *** [/gnu/store/dgf45yi11yn08fq8hm1cp49yqhyg0nl6-glibc-hurd-headers-cross-i686-pc-gnu-2.18/include/gnu/stubs.h] Error 2

and it cannot be built. I am sending you the log as well so you can
see for yourself.