test defs: better detection of cross-compile mode

test defs: better detection of cross-compile mode

* configure.ac (AC_CANONICAL_HOST): New, probably not strictly
necessary, but useful to complements AC_CANONICAL_BUILD.
* tests/defs-static.in ($host_alias, $build_alias): New variables,
exported, user-overridable, defaults AC_SUBST'd by configure.
* tests/defs (cross_compiling): Consider cross-compilation mode
active only if `$host_alias' is not empty *and* it differs from
`$build_alias'.  This is more faithful to the way configure
determines whether it is cross-compiling, but still allows the
user to easily force cross-compilation by overriding `host_alias'
and `build_alias' at runtime.

From a report by Ralf Wildenhues.