[Documentation] Add DUNE_CONTROL_PATH trick to buildsystem.tex.

[[Imported from SVN: r6770]]

doc/buildsystem/buildsystem.tex

@@ -1541,6 +1541,22 @@ you options file the \configure  cache is created. Be aware that the file persis
 between \dunecontrol runs. If you encounter any problems, you want to change the 
 configuration or to rerun all checks, the cache file should be removed manually.
 
+\minisec{Faster module search}
+If the environment variable \texttt{DUNE\_CONTROL\_PATH} is not set, \dunecontrol 
+recursively searches for \dune modules below the working directory. Depending on
+your installation, this search may take quite a bit of time.  It can be
+avoided by including the paths to the \texttt{dune.module} files of all modules in 
+\texttt{DUNE\_CONTROL\_PATH}.
+
+This list of paths can actually be created on the fly by a command.
+Let us assume all \dune modules are in one directory, which is the working directory 
+for \dunecontrol, too. By adding the line
+\begin{lstlisting}[language=make,showstringspaces=false]
+DUNE_CONTROL_PATH="`ls */dune.module | tr '\n' ':'`"
+\end{lstlisting}
+to your opts file, no search will be performed.  Instead, only the faster, non-recursive 
+\texttt{ls} command is called.
+
 \section{Further documentation}
 
 \minisec{automake \& Makefile.am}