yacc: warn about conditional content in *YFLAGS variables This commit fixes automake bug#7804. * automake.in (lang_yacc_target_hook): Warn if any of the relevant *YFLAGS variables has conditional contents (not only a conditional definition). Related refactoring. * NEWS: Updated. * tests/yflags-conditional.test: Updated and extended. * tests/yflags-conditional-force.test: New test. * tests/Makefile.am (TESTS): Updated.