bugfix: makefile not install external/ headers into subdir
bugfix: makefile not install external/ headers into subdir

swansontec/libbitcoin,swansontec/libbitcoin,libtetcoin/libtetcoin-consensus,libmetrocoin/libmetrocoin-consensus,GroestlCoin/libgroestlcoin,Airbitz/libbitcoin,libmetrocoin/libmetrocoin-consensus,swansontec/libbitcoin,swansontec/libbitcoin,BWallet/libbitcoin,Airbitz/libbitcoin,GroestlCoin/libgroestlcoin,libmetrocoin/libmetrocoin-consensus,BWallet/libbitcoin,libmetrocoin/libmetrocoin-consensus,GroestlCoin/libgroestlcoin,libmetrocoin/libmetrocoin-consensus,BWallet/libbitcoin,libtetcoin/libtetcoin-consensus,BWallet/libbitcoin,libtetcoin/libtetcoin-consensus,GroestlCoin/libgroestlcoin,libtetcoin/libtetcoin-consensus,Airbitz/libbitcoin,libtetcoin/libtetcoin-consensus,Airbitz/libbitcoin