title: Implementation of an 'install' target with SCons
body: <p>I try to setup a build system to build a shared lib <em>MySharedLib</em> and a binary <em>MyBinary</em> that rely on <em>MySharedLib</em>. The build system should be able to install <em>MySharedLib</em> and <em>MyBinary</em> when asked for it, and only when asked for it.</p>
  
  <pre><code>import os.path
  
  env = Environment()
  env.Append(CCFLAGS='-g -Wall -ansi -O2')
  
  prefix = '/usr/local'
  lib_dir = os.path.join(prefix, 'lib')
  bin_dir = os.path.join(prefix, 'bin')
  Export('env prefix lib_dir bin_dir')
  
  libMySharedLib = SConscript('libMySharedLib/SConscript')
  MyBinary   = SConscript('MyBinary/SConscript')
  
  env.Alias('install', env.Install(lib_dir, libMySharedLib))
  env.Alias('install', env.Install(bin_dir, MyBinary))
  </code></pre>
  
  <p>When I run SCons with no command line arguments, MySharedLib and MyBinary are built, but it also it try to install them. I want them installed only just when I ask for the <em>install</em> target. I tried many times, read the docs, but I can't figure it out.</p>