Build: configure: let "make dist" tarballs be ustar/posix, not v7 format

This avoids problem with "file name is too long (max 99); not dumped"

• we don't suffer from this currently, but eventually could (see also

the subsequent, related change in the set). Not also that some git
hosting sites (GitHub) will also offer tarballs (effectively our current
official, blessed and authoritative redistributables) using *only* even
newer format (pax), which may effectively cause portability issues(!).