Allow the user to specificy the relative build-dir
Allow the user to specificy the relative build-dir

For certain packages we want to be able to do the build work in directory
separate different from the unpack root.  The 'build-dir' option allows
us to do that.  It's specified relative to the working directory where
the files are unpacked.

jlisee/xpkg,jlisee/xpkg,jlisee/xpkg