title: How to add .org files under org-directory to agenda
body: <p>I am facing the following problem: Org mode has the customizable variable <code>org-agenda-files</code>. The files listed by absolute path under this variable are used for building the org agenda window. If an element in the list is a directory, then all <code>.org</code> files matching the <code>org-agenda-regex</code> are processed to build the agenda view. On the other hand there is also another customizable variable: <code>org-directory</code>. Shockingly though, if <code>org-agenda-files</code> is undefined, the agenda view does not try to use the matching <code>.org</code> files under  <code>org-directory</code>. Also, listing the paths of the agenda files relative to <code>org-directory</code> in <code>org-agenda-files</code> is not accepted, as it complains that the files are not found.</p>
  
  <p>My question is, is there an reasonably elegant way to make use of the <code>org-directory</code> in the lookup of agenda files, either by using it as the default to look for all .org files, or to describe the agenda files list with relative paths to <code>org-directory</code>? </p>