Avoid using empty macro arguments.

These are not supported by all compilers (gcc 2.95 but also older SPARC
compilers, see gcc bug #33304 for example), and there is no real need for them.
One use of this feature remains in libavdevice/v4l2.c which can't be
replaced quite as easily.

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>