* README (Requirements): Remove Nixpkgs. Remove mentions of building
from Git.
(Installation): Refer to the manual.
* HACKING: List requirements for when building from Git.
Remove the `dot: Command not found' error.
* daemon.am (libexec_PROGRAMS, nix_setuid_helper_SOURCES,
nix_setuid_helper_CPPFLAGS, nix_setuid_helper_LDADD): New variables.
* test-env.in: Set and export `NIX_SETUID_HELPER'.
* README (Installing Guix as non-root): New section.
* configure.ac: Change package name to "GNU Guix", and bug-report
address to `gnu-system-discuss@gnu.org'.
* doc/guix.texi: Replace "Guix" by "GNU Guix" in some places.
(Top, Introduction): Mention "for the GNU system".
* HACKING, README: Use "GNU Guix" instead of "Guix" in some places.
* configure.ac: Always show the result of checking for Nixpkgs. Don't
warn when Nixpkgs is not found.
* Makefile.am (AM_DISTCHECK_CONFIGURE_FLAGS): Remove `--with-nixpkgs'
flag.
* guix/utils.scm (%nixpkgs-directory): Use either the compile-time or
the run-time `NIXPKGS' environment variable.
* release.nix (jobs.tarball, jobs.build): Remove `--with-nixpkgs'
configure flag.
* README: Mark Nixpkgs as optional.
* distro/packages/databases.scm, distro/packages/guile.scm,
distro/packages/typesetting.scm: Change uses of `nixpkgs-derivation*'
to `nixpkgs-derivation', to avoid failing at compile-time.
