Replacing string += "." by '.'

[Wolfgang Bangerth]

> >I don't know how to test the patch. In case someone would like to consider 
> >it, please test it according to the your principles before applying it.
> 
> In order to test it,  just run "make check" inside the libstsdc++-v3 
> build directory.

OK, will start a bootstrap. Might take until after lunch (+add time for 
seminar, so will post results in late afternoon).

W.

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Wolfgang Bangerth              email:           bangerth@ticam.utexas.edu
                               www: http://www.ticam.utexas.edu/~bangerth