Update build-unix.md
Update build-unix.md

If a user hasn't ready run autogen.sh, configure doesn't yet exist.  Do the instructions seem to presume you have already built bitcoin?  I don't think they do.

Would stifle beginners.

Update build-unix.md

wastholm/bitcoin,wastholm/bitcoin,wastholm/bitcoin,wastholm/bitcoin,wastholm/bitcoin,wastholm/bitcoin