relaxed requirements on std::string

[Lars Gullik Bjønnes]

I have reduced LyX' requirements on std::string so that we are now
able to compile with the std::string distributed with egcs and gcc
2.95.x. LyX will use this by default. If you want to use the lyxstring
you have to use --with-included-string. One "bad" thing about the
 std::string in gcc is that it enlarges the binary size of LyX with
 some ~800k.

This should make life a lot easier for those that are trying to port
LyX to gtk--.

        Lgb