Make RXGEN_CPPCMD work on unix

Change-Id: Id08d6f02eb117d9d06b6814e39658d69e05bd43b
Reviewed-on: http://gerrit.openafs.org/9963
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>