Revert the fix for PR automake/291:

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.