** Provide header files containing decls for alloca, strings, etc.
** Cross compiling:
*** Error messages include instructions for overriding defaults using
config.site.
*** Distribute a config.site corresponding to a hypothetical bare POSIX system with c89.
*** Cache consistency checking: ignore cache if environment
(CC or PATH) differs.
** Site defaults:
*** Convention for consistency checking of env vars and options in config.site so config.site can print obnoxious messages if it doesn't like options or env vars that users use.