tests: avoid spurious failures in tests on C++ and lex

tests: avoid spurious failures in tests on C++ and lex

This is a follow-up to commit v1.11-2128-g7f2bc63 of 09-04-2012,
"tests: avoid spurious failures with non-flex 'lex' programs and
C++".  It is required to avoid a couple of spurious failures on
Solaris and NetBSD systems (at least).  See also automake bug#11185.

* t/lex-clean-cxx.sh (parsefoo.lxx): Do not declared the provided
dummy 'isatty' function as 'static', since that might conflict with
a declaration of it as 'extern' pulled in through other system
* t/lex-depend-cxx.sh (joe.ll): Likewise.
* t/README: Update advice, to avoid similar issues in the future.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>