The following macros make the system type available to configure
scripts.
The variables ‘build_alias’, ‘host_alias’, and
‘target_alias’ are always exactly the arguments of --build,
--host, and --target; in particular, they are left empty
if the user did not use them, even if the corresponding
AC_CANONICAL macro was run. Any configure script may use these
variables anywhere. These are the variables that should be used when in
interaction with the user.
If you need to recognize some special environments based on their system type, run the following macros to get canonical system names. These variables are not set before the macro call.
Compute the canonical build-system type variable, build, and its
three individual parts build_cpu, build_vendor, and
build_os.
If --build was specified, then build is the
canonicalization of build_alias by config.sub,
otherwise it is determined by the shell script config.guess.
Compute the canonical host-system type variable, host, and its
three individual parts host_cpu, host_vendor, and
host_os.
If --host was specified, then host is the
canonicalization of host_alias by config.sub,
otherwise it defaults to build.
Compute the canonical target-system type variable, target, and its
three individual parts target_cpu, target_vendor, and
target_os.
If --target was specified, then target is the
canonicalization of target_alias by config.sub,
otherwise it defaults to host.
Note that there can be artifacts due to the backward compatibility code. See Hosts and Cross-Compilation, for more.