[Dune] License link broken

[Carsten Graeser]

Dear Dune Developers,
the link to the libstc++ license (GPL2 + runtime exception) in the
README files is broken. The new link seems to be

http://gcc.gnu.org/onlinedocs/libstdc++/manual/bk01pt01ch01s02.html

Carsten
-- 
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Carsten Gräser           | phone: +49-30 / 838-75349
Freie Universität Berlin | fax  : +49-30 / 838-54977
Institut für Mathematik  | email: graeser at math.fu-berlin.de
Arnimallee 6             |
14195 Berlin, Germany    | URL  : http://page.mi.fu-berlin.de/graeser
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