set package vendor to smanders only if not defined
set package vendor to smanders only if not defined

* allow projects that use externpro the ability to set CPACK_PACKAGE_VENDOR to something other than smanders
* see issue https://github.com/smanders/externpro/issues/127
* CPACK_PACKAGE_VENDOR needs to be set before proExecuteStep() is called, which calls proSetCpackOpts()

smanders/externpro