##### 6.3.2.2 The `config.build`; `config.host`; and `config.gcc` Files

The `config.build` file contains specific rules for particular systems which GCC is built on. This should be used as rarely as possible, as the behavior of the build system can always be detected by autoconf.

The `config.host` file contains specific rules for particular systems which GCC will run on. This is rarely needed.

The `config.gcc` file contains specific rules for particular systems which GCC will generate code for. This is usually needed.

Each file has a list of the shell variables it sets, with descriptions, at the top of the file.

FIXME: document the contents of these files, and what variables should be set to control build, host and target configuration.