The section on config.h copied here from the grid howto. And fix a few mistakes while I'm here

[[Imported from SVN: r5557]]

doc/buildsystem/buildsystem.tex

@@ -696,7 +696,7 @@ the web page. This conversion can be done using inkscape
 \label{configure.ac}
 
 \configureac  is a normal text file that contains several \autoconf
-macros. These macros are evaluated my the \texttt{m4} macro processor
+macros. These macros are evaluated by the \texttt{m4} macro processor
 and transformed into a shell script.
 
 \begin{lst}[File dune-common/configure.ac] \mbox{}
@@ -732,7 +732,7 @@ We offer a set of macros that can be used in your \configureac:
   \texttt{DUNE\_CHECK\_ALL}.
 \end{itemize}
 
-\texttt{DUNE\_CHECK\_ALL} and \texttt{DUNE\_CHECK\_ALL\_M} define certain
+\texttt{DUNE\_CHECK\_ALL} defines certain
 variables that can be used in the \configure script or in the
 \makefileam:
 
@@ -743,21 +743,44 @@ variables that can be used in the \configure script or in the
 \item \texttt{DUNE\textit{\,MODULE\,}ROOT}
 \end{itemize}
 
-The last step to a complete \configureac is that you tell autoconf
-which files should be generated by \configure. Therefor you add an
+The last step to a complete \configureac is that you tell \autoconf
+which files should be generated by \configure. Therefore you add an
 \texttt{AC\_CONFIG\_FILES([\textit{WhiteSpaceSeparatedListOfFiles}])}
 statement to your \configureac. The list of files should be the list
-of files that are to be generated, not the input -- i.e. you would
+of files that are to be generated, not the input---i.e. you would
 write
 \begin{lstlisting}[language=make]
 AC_CONFIG_FILES([Makefile doc/Makefile])
 \end{lstlisting}
-end not
+instead of
 \begin{lstlisting}[language=make]
 AC_CONFIG_FILES([Makefile.in doc/Makefile.in])
 \end{lstlisting}
 After you told \autoconf which files to create you have to actually
-trigger their creation with command \texttt{AC\_OUTPUT}
+trigger their creation with command \texttt{AC\_OUTPUT}.
+
+\subsection{Using configuration information provided by configure}
+
+The \lstinline!./configure! script in the module produces a file
+\lstinline!config.h!\ that contains information about the configuration
+parameters, for example which of the optional grid implementations is
+available and which dimension has been selected (if applicable). This
+information can then be used at compile-time to include header files
+or code that depend on optional packages.
+
+As an example, the macro \lstinline!HAVE_UG!\ can be used to compile
+UG-specific code as in
+\begin{lstlisting}[basicstyle=\ttfamily\scriptsize]
+#if HAVE_UG
+#include"dune/grid/uggrid.hh"
+#endif
+\end{lstlisting}
+
+It is important that the file \lstinline!config.h! is the first
+include file in your application!
+
+
+
 
 \subsection{dune-autogen}
 \label{autogen}