subdir-objects: complain if it isn't enabled Since the next major automake version will make the behaviour so far only activated with the 'subdir-object' option mandatory, it's better if we start warning users not using that option. As suggested by Peter Johansson, we strive to avoid the warning when it would be irrelevant, i.e., if all source files sit in "current" directory. See automake bug#13378. * automake.in (handle_single_transform): Print the warning when necessary. * t/subobj.sh: Enhance. * t/ax/depcomp.sh: Adjust. * t/cscope.tap: Likewise. * t/depcomp8a.sh: Likewise. * t/depcomp8b.sh: Likewise. * t/ext2.sh: Likewise. * t/extra-portability.sh: Likewise. * t/fort2.sh: Likewise. * t/fort4.sh: Likewise. * t/fort5.sh: Likewise. * t/lex-line.sh: Likewise. * t/libtool3.sh: Likewise. * t/ltinstloc.sh: Likewise. * t/ltlibsrc.sh: Likewise. * t/ltorder.sh: Likewise. * t/parallel-tests-suffix-prog.sh: Likewise. * t/sourcefile-in-subdir.sh: Likewise. * t/specflg9.sh: Likewise. * t/subobj4.sh: Likewise. * t/subobj7.sh: Likewise. * t/subpkg-yacc.sh: Likewise. * t/subpkg.sh: Likewise. * t/suffix-custom-subobj-and-specflg.sh: Likewise. * t/vala-libs.sh: Likewise. * t/vala-non-recursive-setup.sh: Likewise. * t/yacc-grepping2.sh: Likewise. * t/yacc-line.sh: Likewise.