Trailing X=Y arguments are supposed to be passed through unchanged
to bdb's configure. This was not the case, at least with OpenBSD
Fix this by not storing the arguments in a temporary variable but
passing "$@" through directly.
Instead of maintaining not-easily-tested instructions for building BerkeleyDB
in doc/build-unix.md, package the installation as a script in contrib/. This
allows shared usage from a number of contexts, e.g. Docker.
Thanks to @jonasschnelli, @laanwj for feedback.