From autoconf docs:
> ‘$target’ is for use by a package creating a compiler or similar.
> For ordinary packages it's meaningless and should not be used.
Since GIMP is not a compiler, nor anything similar, let's not make
anything from this information.
Thanks to Quentin Glidic for reporting the issue (on libmypaint bug
tracker, since it was using nearly the same code there too).

This commit also improves configure output regarding host detection,
and uses dedicated canonical variables $host_cpu/os instead of the full
value.