Daniel Wyatt authored Jun 23, 2018 and Brad King committed Jun 26, 2018

Ensure we use the `cmake` corresponding to the running `cpack`
even if it is not first in `PATH` or has had its name changed.
This was accidentally left out in commit v3.7.0-rc1~81^2 (CPack/RPM:
Generate source rpm (SRPM) packages on demand, 2016-09-19).