refactor: processing of input makefile rules This is a pure refactoring, with no intended functional or semantic changes. It breaks up an overly-long function in three smaller sub-functions. This change will very especially useful for the work on Automake-NG. * lib/Automake/Rule.pm (define): Move quite a lot of code out, into ... (_rule_defn_with_exeext_awareness, _maybe_warn_about_duplicated_target, _conditionals_for_rule): ... these new subroutines. Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>