By default, rustc use `cc` as linker. Under OpenBSD, `cc` is gcc version 4.2.1.
So use the compiler found at configure-time for linking: it will be gcc 4.9.
It permits to resolv problem of finding -lestdc++ or -lgcc. For base gcc (4.2), there are in not standard path, whereas for ports gcc (4.9) there are in standard path.
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