Add small precision in PETSc install doc.

Configure option `--with-fortran=0` is apparently gone in PETSc 3.5.
See http://www.mcs.anl.gov/petsc/documentation/installation.html.

doc/installation/installation.tex

@@ -314,7 +314,13 @@ \subsubsection{Installing PETSc from source}
 configuration option.  Using shared libraries may be unwise on certain clusters,
 etc.; check with your system administrator.
 
-\item It is sometimes convenient to have PETSc grab a local copy of BLAS\index{BLAS (\emph{Basic Linear Algebra Subsystem})} and LAPACK\index{LAPACK (\emph{Linear Algebra PACKage})} rather than using the system-wide version.  So one may add ``\texttt{--with-fortran=0} \texttt{--download-f2cblaslapack=1}'' to the other configure options; note there is no use of Fortran in PISM.
+\item It is sometimes convenient to have PETSc grab a local copy of
+BLAS\index{BLAS (\emph{Basic Linear Algebra Subsystem})} and
+LAPACK\index{LAPACK (\emph{Linear Algebra PACKage})} rather than using the
+system-wide version. So one may add ``\texttt{--download-f2cblaslapack=1}'' to
+the other configure options. Since there is no use of Fortran in PISM, Fortran
+can also be disabled using ``\texttt{--with-fortran=0}'' (PETSc~$\le 3.4$),
+or ``\texttt{--with-fc=0}'' (PETSc~$\ge 3.5$).
 
 \item If there is an existing MPI\index{MPI (\emph{Message Passing Interface})} installation, for example at \texttt{/home/user/mympi/}, one can point PETSc to it by adding the option ``\texttt{--with-mpi-dir=/home/user/mympi/}''.  The path used in this option must have MPI executables \texttt{mpicxx} and \texttt{mpicc}, and either \texttt{mpiexec} or \texttt{mpirun}, in sub-directory \texttt{bin/} and MPI library files in sub-directory \texttt{lib/}. If you get messages suggesting that PETSc cannot configure using your existing MPI, you might want to try adding the \texttt{--download-mpich=1} (or \texttt{--download-openmpi=1}) option to PETSc's configure command.