Revert the fix for PR automake/291: * lib/Automake/Variable.pm (define): Do not warn about variables starting with `_' which are an issue on NEWS-OS 4.2R. According to Paul Eggert, NEWS-OS 4.2R is ten years old, the latest version was released on 1996 and is not in wide use. On the other hand, variables starting with `_' are mandatory in several situations, so diagnosing them is more annoying than useful. * tests/canon5.test: Do not use -Wno-portability. * tests/vars2.test: Delete. * tests/Makefile.am (TESTS): Remove vars2.test. Suggested by Jim Meyering.