Build: Don't require sudo for `make tarball`
The whole point of `make tarball` is to make it easy for users to create
a binary distribution of libjpeg-turbo on platforms that aren't
supported by our official build system, so requiring root permissions
somewhat defeated that purpose. Intead, the script now attempts to
detect whether the system has GNU tar or a recent version of BSD tar
that supports setting the ownership of the files in the tarball.