Added check for variable CONFIGURE_DEFINES into options.c

Added check for variable CONFIGURE_DEFINES into options.c

The file containing CONFIGURE_DEFINES variable, configure.h, is not present if
openvpn is built using the Python + Visual C -based buildsystem. This causes the
build to fail. This patch adds a check to see if variable exists before trying
to use it.

Signed-off-by: Samuli Seppänen <samuli@openvpn.net>
Acked-by: Peter Stuge <peter@stuge.se>
Signed-off-by: David Sommerseth <dazo@users.sourceforge.net>