Remove superfluous override directive in make check

Remove superfluous override directive in make check

Travis CI and OS X system make on 10.9.x at least don't have GNU make
>=3.82 which contains a parser enhancement that allows multiple
directives.

Signed-off-by: Gerd Flaig <gefla@pond.sub.org>
Here's why removing override is a good idea.  The variable assignment
should already override anything Make may find in its environment.
All "override" does is protect against unwise make arguments.

"info make" says:

    The `override' directive was not invented for escalation in the war
    between makefiles and command arguments.  It was invented so you can
    alter and add to values that the user specifies with command
    arguments.

Thus, override is ill-advised here.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>