uname reports amd64.
* Drop the FreeBSD version number, e.g. "i686-freebsd" instead of
  "i686-freebsd8.2".

This should be more robust and also plays better with
cross-compilation---it uses the host name, instead of using the build
name.

  intermittent problems are gone by now.  WAL mode is preferrable
  because it does way fewer fsyncs.

  the Nix:: namespace.

  bindings to be used in Nix's own Perl scripts.

  The only downside is that Perl XS and Automake/libtool don't really
  like each other, so building is a bit tricky.

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  location to DBI and DBD::SQLite can be passed with --with-dbi and
  --with-dbd-sqlite.

  little RAM.  Even if the memory isn't actually used, it can cause
  problems with the overcommit heuristics in the kernel.  So use a VM
  space of 25% of RAM, up to 384 MB.

  returned by pkg-config.

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  --enable-gc is given.

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  because Berkeley DB needed it on some platforms, but we don't use
  BDB anymore.

  On FreeBSD, if you link against pthreads, then the main thread gets
  a 2 MB stack which cannot be overriden (it ignores "ulimit -s"):

    http://www.mail-archive.com/freebsd-hackers@freebsd.org/msg62445.html
  
  This is not enough for Nix.  For instance, the garbage collector can
  fail if there is a pathologically deep chain of references
  (http://hydra.nixos.org/build/556199).  2 MB is also not enough for
  many Nix expressions.
  
  Arguably the garbage collector shouldn't use recursion, because in
  NixOS unprivileged users can DOS the garbage collector by creating a
  sufficiently deeply nested chain of references.  But getting rid of
  recursion is a bit harder.

  faster than the old mode when fsyncs are enabled, because it only
  performs an fsync() when doing a checkpoint, rather than at every
  commit.  Some timings for doing a "nix-instantiate /etc/nixos/nixos
  -A system" after modifying the stdenv setup script:

    42.5s - SQLite 3.6.23 with truncate mode and fsync
     3.4s - SQLite 3.6.23 with truncate mode and no fsync
    32.1s - SQLite 3.7.0 with truncate mode and fsync
    16.8s - SQLite 3.7.0 with WAL mode and fsync, auto-checkpoint
            every 1000 pages
     8.3s - SQLite 3.7.0 with WAL mode and fsync, auto-checkpoint
            every 8192 pages
     1.7s - SQLite 3.7.0 with WAL mode and no fsync

  The default is now to use WAL mode with fsyncs.  Because WAL doesn't
  work on remote filesystems such as NFS (as it uses shared memory),
  truncate mode can be re-enabled by setting the "use-sqlite-wal"
  option to false.

This reverts commit 22405. Apparently, these programs aren't necessarily
required when building from a release archive.

The build fails if these tools aren't available.

I copied the configure.ac code about CC_FOR_BUILD from libX11.

  http://hydra.nixos.org/build/311170).

  --enable-shared.
* In libutil/libstore/libexpr etc., link against sqlite and aterm.
* Some more header file hygiene.

* Don't refer to config.h in util.hh, because config.h is not
  installed (http://hydra.nixos.org/build/303053).