maint.mk: fix "release" target to build _version

maint.mk: fix "release" target to build _version

This fixes a bug in README-release whereby following the outlined
steps, one would publish a tarball whose programs would report
--version output not consistent with the package version number.
This bug caused grep-2.15 to produce a grep program whose
--version option made it print 2.14.56-1e3d rather than 2.15.
* top/maint.mk (release): Making this target build "_version"
ensures that the new version number is reflected in configure.