title: Make distcheck doesn't copy all sources
body: <p>My project is structured as followed:</p>
  
  <p>/
      subdir1
      subdir2
      subdir3
      common_include</p>
  
  <p>Each subdir compiles different binaries and libs for my project.
  common_include includes .h (C++ headers) included by all my subdirs.</p>
  
  <p>These headers files are NOT added to each Makefile.am in the SOURCES attribute. 
  But I add the following path -I../common_include to the CPPFLAGS attribute</p>
  
  <p>I use GNU build system for configuration. Everything is working.</p>
  
  <p>But when I want to release the project by typing: make distcheck in order to create: my-project-1.0.tar.gz, the archive is successfully created.</p>
  
  <p>But it contains all subdirs and not the common_include directory. So the final user can't compile..</p>
  
  <p>Any help ?</p>
  
  <p>Thank you</p>