build_vms.com: detect and use zlib shared image

Update the build_vms.com to detect and use zlib shared image installed
by the ZLIB kit produced by Jean-Francois Pieronne, and the also the
future ZLIB 1.2.8 kit in addition to the older ZLIB kits.

Also fix the indentation to match one of the common standards used for
VMS DCL command files and removed the hard tab characters.

Tested on OpenVMS 8.4 Alpha and IA64, and OpenVMS 7.3 VAX.