yacc: support variable expansions in *YFLAGS definition. This change fixes automake bug#7800. * automake.in (lang_yacc_target_hook): Use 'value_as_list_recursive' instead of 'variable_value' to get the value of *YFLAGS variables. Related changes. ($DASH_D_PATTERN): Removed. * tests/Makefile.am (XFAIL_TESTS): Remove yflags-var-expand.test. * tests/yacc-clean.test: Remove workaround for now-fixed bug. * NEWS: Update.