autotools: Use AC_CANONICAL_HOST

We were calling AC_CANONICAL_TARGET in configure.ac under the mistaken impression that this allowed setting the target architecture for cross-compilation output.

However, in GNU terminology the system type that this the target of cross-compilation is the "host" and "target" is reserved for setting what the output of the compiled application should itself target, when compiling a cross-compiler. Finally "build" is the system type the build system is itself running under.

So to cross-compile, one invokes ./configure --host ... not --target.

In any case, we only make use of the host variable, so that is the macro we need to invoke to provide it.

Thanks to Maarten Bosmans for reporting the issue.