Trailing X=Y arguments are supposed to be passed through unchanged
to bdb's configure. This was not the case, at least with OpenBSD
6.2's shell.
Fix this by not storing the arguments in a temporary variable but
passing "$@" through directly.
Replace the clang patch with a new and improved version that also fixes
the build issues with OpenBSD and FreeBSD's clang, and apply it
unconditionally.
This needs testing on OSX.
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.
