* Cleaned up the semantics of Fix expressions.

* Conditionals and variables in Fix expressions.  This allows, e.g.,

    Descr(
    [ Bind("pkgId", "subversion-0.21.0")

    , Bind("httpsClient", Bool(True))
    , Bind("httpServer", Bool(True))

    , Bind("ssl", If(Var("httpsClient"), Fix("./openssl-0.9.7b.fix"), ""))

    , Bind("httpd", If(Var("httpServer"), Fix("./httpd-2.0.45.fix"), ""))
    ...
    ])

  which introduces domain feature variables httpsClient and httpServer
  (i.e., whether Subversion is built with https client and webdav
  server support); the values of the variables influences package
  dependencies and the build scripts.

  The next step is to allow that packages can express constraints on
  each other.  E.g., StrategoXT is dependent on an ATerm library with
  the "gcc" variant enabled.  In fact, this may cause several
  Nix instantiations to be created from a single Fix descriptor.  If
  possible, Fix should try to find the least set of instantiations
  that obeys the constraints.

test/build/openssl-build.sh

@@ -0,0 +1,12 @@
+#! /bin/sh
+
+export PATH=/bin:/usr/bin
+
+top=`pwd`
+tar xvfz $src || exit 1
+cd openssl-* || exit 1
+./config --prefix=$top || exit 1
+make || exit 1
+make install || exit 1
+cd $top || exit 1
+rm -rf openssl-* || exit 1