autotools: Remove AC_CANONICAL_TARGET
We were adding the --target configure option in the mistaken belief that it was useful for cross-compiling. In GNU terminology, "target" refers to the architecture the programme being compiled should itself target, as when building a cross-compiler.
To specify the platform the build itself should target one instead uses --host, while --build refers to the system where the build itself it taking place.
We were refering to the correct host* variables inside
configure.ac. The call to AC_CANONICAL_TARGET is simply
unnecessary.