Fix wheezy libstdc not found error

The nova-guest does not run due to the following error:

nova-guest: /usr/lib/libstdc++.so.6: version `GLIBCXX_3.4.15' not
found (required by nova-guest)

Since wheezy comes with a newer version of gcc, we can use a new
gcc flag '-static-libstdc++' introduced in 4.5.

Also the specific gcc version path was removed.