CMake´s buildscript generator to use (e.g. Unix Makefiles).
Possible generators include:

The build tool chosen by CMake — corresponding to the generator — is exposed in the CMAKE_BUILD_TOOL build environment variable. Subsequent build steps may use $CMAKE_BUILD_TOOL} to execute the generated scripts.
Its actual value is retrieved from the generated CMake cache file (CMakeCache.txt). Usually that value is something like /usr/bin/gmake on Linux.