This allows many packages to build in a chroot that lacks /bin and
thus /bin/sh.
* guix/build/gnu-build-system.scm (patch-source-shebangs): New
procedure.
(%standard-phases): Add it.
* guix/build/utils.scm (executable-file?): New procedure.
* distro/packages/perl.scm (perl): Don't use /bin/sh to run `Configure'.