A supported C compiler, linker, librarian and make are required since xHarbour uses C as intermediate language. The supported compilers/tools are:
DJ Delorie's DJGPP
DJGPP is a complete 32-bit C/C++ development system for Intel 80386 (and higher) PCs running DOS. It includes ports of many GNU development utilities. The development tools require a 80386 or newer computer to run, as do the programs they produce. In most cases, the programs it produces can be sold commercially without license or royalties.
OpenWatcom
In a single package, Open Watcom C/C++ provides a comprehensive development environment with the tools, SDK's, and libraries you need to create powerful 16- and 32- bit applications for popular PC platforms.
gnu make
The gnu make tool.