Resolve relative CPACK_PROJECT_CONFIG_FILE explicitly at config
generation time. Otherwise, it will be resolved at runtime
relative to the CPack execution directory (which could be anything).
Additionally, issue a warning if reading PACK_PROJECT_CONFIG_FILE
fails at runtime.
Fixes: #15522
In commit da5ac4bb60 (cpack: Add `CPACK_INSTALL_CMAKE_CONFIGURATIONS`
variable, 2019-07-10, v3.16.0-rc1~165^2) we added support for running
the `cpack -C ...` command-line with multiple configurations. Add an
explicit test for this.
Fixes: #19918
In commit v3.11.0-rc1~68^2 (CPack: accept --trace and --trace-expand,
2017-12-09) a nullptr dereference was added that occurs when
`cpack -G NotAGenerator` is invoked. Add the needed condition.
Fixes: #17900