tests: a test about warnings in category 'obsolete' was failing spuriously * t/warnings-obsolete-default.sh : After our removal of $(INCLUDES) support, this test has been spuriously failing because it was assuming that a definition of the INCLUDES make variable only caused a warning rather than a fatal error. Fix it to use a definition of the newly obsolescent ACLOCAL_AMFLAGS make variable. Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>