Restrict visibility flags to gcc.
Restrict visibility flags to gcc.

tuomassiren/libbitcoin-explorer,GroestlCoin/libgroestlcoin-explorer,libmetrocoin/libmetrocoin-explorer,GroestlCoin/libgroestlcoin-explorer,libtetcoin/libtetcoin-explorer,cancoin/libbitcoin-explorer,tuomassiren/libbitcoin-explorer,cancoin/libbitcoin-explorer,libtetcoin/libtetcoin-explorer,GroestlCoin/libgroestlcoin-explorer,libmetrocoin/libmetrocoin-explorer