Without the change builda fails on `master` as
https://hydra.nixos.org/build/238481288/nixlog/3:
checking for zlib - version >= 1.2.3... 1.3, bad version string given
by zlib, sometimes due to very old zlibs that didnt correctly
define their version. Please upgrade if you are running an
old zlib... no
The failure happens due to 2-digit zlib version string.
Add a trivial change to allow 2-digit support in addition to existing
3-digit ones..
