Fix -Wunused-variable warning on SKIP_ACE
Remove YARP_CMAKE_CONFIG define

It looks like something left from the past, when yarp could be build
either with CMake or with autotools, but now it is just enabled, without
any check.

Also since now yarp/conf/version.h is always included, it makes no sense
checking if YARP_VERSION is defined.
Add YARP_UNUSED macro to suppress unused variable warnings
Remove ACE_UNUSED_ARG and use YARP_UNUSED instead
Replace (void)x with YARP_UNUSED
Fix -Wunused-but-set-variable warnings