sourcebuild.texi (Subdirectories): Document common subdirectory.

	* doc/sourcebuild.texi (Subdirectories): Document common
	subdirectory.

From-SVN: r189988

gcc/ChangeLog

+2012-07-30  Joseph Myers  <joseph@codesourcery.com>
+
+	* doc/sourcebuild.texi (Subdirectories): Document common
+	subdirectory.
+
 2012-07-30  John David Anglin  <dave.anglin@nrc-cnrc.gc.ca>
 
 	PR middle-end/53823

gcc/doc/sourcebuild.texi

@@ -176,6 +176,12 @@ those for other languages are not.  @xref{Front End, ,
 Anatomy of a Language Front End}, for details of the files in these
 directories.
 
+@item common
+Source files shared between the compiler drivers (such as
+@command{gcc}) and the compilers proper (such as @file{cc1}).  If an
+architecture defines target hooks shared between those places, it also
+has a subdirectory in @file{common/config}.  @xref{Target Structure}.
+
 @item config
 Configuration files for supported architectures and operating
 systems.  @xref{Back End, , Anatomy of a Target Back End}, for