yacc: more tests on *YFLAGS support * tests/yflags-var-expand.test: New test, still xfailing. It exposes automake bug#7800 -- "automake fails to honor `-d' in AM_YFLAGS when variable expansions are involved". * tests/yflags-d-false-positive.test: New test, checking that automake do not spuriously see `-d' in *YFLAGS when that isn't really there. * tests/yflags-force-override.test: New test, checking that automake can cope with definition of the YFLAGS variable in Makefile.am (even if that is an extremely bad practice, as that variable is user-reserved). * tests/yflags-cmdline-override.test: New test, checking that automake can cope with user-redefinition of YFLAGS at configure time and/or at make time. * tests/yflags-conditional.test: New test, checks that automake warns on conditionally-defined *YFLAGS variables. * tests/Makefile.am (TESTS, XFAIL_TESTS): Update.