tests: skip -Wall -Werror with Tru64 cc in cwrapper test.

tests: skip -Wall -Werror with Tru64 cc in cwrapper test.

* tests/cwrapper.at (cwrapper for uninstalled executables):
Use a link test rather than a compile test to flesh out unknown
compiler flags starting with -W; Tru64/OSF1 cc will happily
accept `-Wall -Werror' as indication to pass flags on to
subprocesses.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>