From IFWiki

vbccz is a back end for Volker Barthelmann's VBCC C compiler that targets the Z-machine, and was written by David Given. In other words, instead of using Inform, vbccz lets you write a program in C and compile it for the Z-machine.

Because of licensing restrictions, if you want to use vbccz, you need to download the unaltered VBCC and the vbccz patch separately, then put them together with a suitable patch program.

One project that used vbccz was Silicon Castles (David Given).