Each version inherits from the last one only if they have the same
nonnumeric prefix, i.e. GLIBC_x.y and GLIBC_x.z or FOO_x and FOO_y
but not GLIBC_x and FOO_y.
* scripts/firstversions.awk: Handle libraries that don't have each
particular version named in the third column of shlib-versions.