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Linux: link mysql2 extension to libstdc++

I can't find a way to statically link it to libstdc++ and libgcc_s.
However, considering that libstdc++ has been ABI backward compatible
for 9 years[1], it is probably ok to dynamically link to it.
It seems the story with libgcc_s is similar.

Closes GH-21.

[1] http://stackoverflow.com/a/14082540/20816