The Clozure CL kernel can be built with the following widely available tools:

In general, the more recent the versions of those tools, the better; some versions of gcc 3.x on Linux have difficulty compiling some of the kernel source code correctly (so gcc 4.0 should be used, if possible.) On Mac OS X, the versions of the tools distributed with Xcode should work fine; on Linux, the versions of the tools installed with the OS (or available through its package management system) should work fine if they're "recent enough". On FreeBSD, the installed version of the m4 program doesn't support some features that the kernel build process depends on; the GNU version of the m4 macroprocessor (called gm4 on FreeBSD) should be installed.