title: SCons does not find header files when using variant_dir
body: <p>Hello I running a problem with the use of relative paths and changes in the variant_dir
  I have a hierarchical SCons construction. From a main SConstruct I'm calling sub SConcripts representing the layers of
  my project:</p>
  
  <pre><code>mcal_build_dir              = os.path.join(env.subst('$OUTPUT_DIR'), 'objs', 'MCAL')
  infra_build_dir             = os.path.join(env.subst('$OUTPUT_DIR'), 'objs', 'INFRA')
  
  mcal_path             = os.path.join('../../Targets/TC275/MCAL/SConscript')
  infra_path            = os.path.join('../../INFRA/SConscript')
  
  
  mcal_objs = SConscript(mcal_path, exports='env env_base', variant_dir=mcal_build_dir, duplicate=0)
  infra_objs = SConscript(infra_path, exports='env env_base', variant_dir=infra_build_dir, duplicate=0)
  </code></pre>
  
  <p>inside the INFRA/Sconscript</p>
  
  <pre><code>includes = [
    '../MCAL/api',
    ........
    ........
  ]
  # SOURCE FILES
  sources = [
      'src/ECU_StartupTask.c',
    ....
  ]
  for include in includes:
      own_env.Append(CPPPATH=[Dir(include).abspath])
  </code></pre>
  
  <p>When it compiles it the header files in MCAL/api are found:</p>
  
  <pre><code>gcc ....... -fno-peephole2 -D_GNU_C_TRICORE_=1 -Ioutput\objs\MCAL\api -IC:\Repositories\fcm3_ssb_sk\Targets\TC275\MCAL\api -Ioutput\objs\ASW\swc_PyroControl\code\api .....
  </code></pre>
  
  <p>now I need to have 2 different build flavours for the mcal, so I tried to perform 2 compilations of the same code in a
  different build dir</p>
  
  <pre><code>mcal_build_dir              = os.path.join(env.subst('$OUTPUT_DIR'), 'objs', 'MCAL/hw_3x')
  mcal_build_dir2                 = os.path.join(env.subst('$OUTPUT_DIR'), 'objs', 'MCAL/hw_4x')
  
  The point is that only changing this the INFRA SConscript does not compile, the headers MCAL/api are not found. 
  
  gcc ....... -fno-peephole2 -D_GNU_C_TRICORE_=1 -Ioutput\objs\MCAL\api -Ioutput\objs\ASW\swc_PyroControl\code\api .....
  </code></pre>
  
  <p>""" note that the absolute path to the MCAL\api is not added in the compilation line """</p>
  
  <p>So it seems that for some reason the absolute path to -IC:\Repositories\fcm3_ssb_sk\Targets\TC275\MCAL\api is not found.
  I don't understand the relation of  the INFRA/SConstruct with the mcal_build_dir change. Is not suppose to be
  independent? I mean when I build the INFRA layer I'm using includes as relative paths to the INFRA/SConscript. 
  I think that when you change the build_dir the SConscript directory is <em>copied</em> to the build_dir and compiled there, but
  what about the header files out of the INFRA layer itself? are they copied? how does the INFRA/SConscript know the
  absolute path to MCAL/api the first time and not the second time.</p>