subdir-objects: complain if it isn't enabled

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.