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  without limitation the rights to use, copy, modify, merge, publish,
  distribute, sublicense, and/or sell copies of the Software, and to
  permit persons to whom the Software is furnished to do so, subject to
  the following conditions:

  The above copyright notice and this permission notice shall be included
  in all copies or substantial portions of the Software.

  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY
  KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE
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  NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
  LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
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<!--

=head1 Variant builds

=head2 Variations on a theme

Other variations of this model are possible. For example, you might decide
that you want to separate out your include files into platform dependent and
platform independent files. In this case, you'd have to define an
alternative to C<$INCLUDE> for platform-dependent files. Most F<Conscript>
files, generating purely platform-independent include files, would not have
to change.

You might also want to be able to compile your whole system with debugging
or profiling, for example, enabled. You could do this with appropriate
command line options, such as C<DEBUG=on>. This would then be translated
into the appropriate platform-specific requirements to enable debugging
(this might include turning off optimization, for example). You could
optionally vary the name space for these different types of systems, but, as
we'll see in the next section, it's not B<essential> to do this, since Cons
is pretty smart about rebuilding things when you change options.

-->

  <para>

  The &variant_dir; keyword argument of
  the &SConscript; function provides everything
  we need to show how easy it is to create
  variant builds using &SCons;.
  Suppose, for example, that we want to
  build a program for both Windows and Linux platforms,
  but that we want to build it in a shared directory
  with separate side-by-side build directories
  for the Windows and Linux versions of the program.

  </para>

  <programlisting>
    platform = ARGUMENTS.get('OS', Platform())

    include = "#export/$PLATFORM/include"
    lib = "#export/$PLATFORM/lib"
    bin = "#export/$PLATFORM/bin"

    env = Environment(PLATFORM = platform,
                      BINDIR = bin,
                      INCDIR = include,
                      LIBDIR = lib,
                      CPPPATH = [include],
                      LIBPATH = [lib],
                      LIBS = 'world')

    Export('env')

    env.SConscript('src/SConscript', variant_dir='build/$PLATFORM')
  </programlisting>

  <para>

  This SConstruct file,
  when run on a Linux system, yields:

  </para>

  <screen>
    % <userinput>scons -Q OS=linux</userinput>
    Install file: "build/linux/world/world.h" as "export/linux/include/world.h"
    cc -o build/linux/hello/hello.o -c -Iexport/linux/include build/linux/hello/hello.c
    cc -o build/linux/world/world.o -c -Iexport/linux/include build/linux/world/world.c
    ar rc build/linux/world/libworld.a build/linux/world/world.o
    ranlib build/linux/world/libworld.a
    Install file: "build/linux/world/libworld.a" as "export/linux/lib/libworld.a"
    cc -o build/linux/hello/hello build/linux/hello/hello.o -Lexport/linux/lib -lworld
    Install file: "build/linux/hello/hello" as "export/linux/bin/hello"
  </screen>

  <para>

  The same SConstruct file on Windows would build:

  </para>

  <screen>
    C:\><userinput>scons -Q OS=windows</userinput>
    Install file: "build/windows/world/world.h" as "export/windows/include/world.h"
    cl /Fobuild\windows\hello\hello.obj /c build\windows\hello\hello.c /nologo /Iexport\windows\include
    cl /Fobuild\windows\world\world.obj /c build\windows\world\world.c /nologo /Iexport\windows\include
    lib /nologo /OUT:build\windows\world\world.lib build\windows\world\world.obj
    Install file: "build/windows/world/world.lib" as "export/windows/lib/world.lib"
    link /nologo /OUT:build\windows\hello\hello.exe /LIBPATH:export\windows\lib world.lib build\windows\hello\hello.obj
    embedManifestExeCheck(target, source, env)
    Install file: "build/windows/hello/hello.exe" as "export/windows/bin/hello.exe"
  </screen>

  <!--

  <scons_example name="ex_var2">
    <file name="SConstruct" printme="1">
    env = Environment(OS = ARGUMENTS.get('OS'))
    for os in ['newell', 'post']:
        SConscript('src/SConscript', variant_dir='build/' + os)
    </file>
  </scons_example>

  <scons_output example="ex_var2">
    <scons_output_command>scons -Q</scons_output_command>
  </scons_output>

  -->
