Add a RunCMake.GNUInstallDirs test with cases covering various install
prefixes. Hard-code the architecture information. Tolerate all
platform-specific LIBDIR values.
Currently the root prefix is not handled well, but verify the current
behavior in the test anyway. This can be addressed with a future
change.
Inspired-by: Alex Turbov <i.zaufi@gmail.com>
5f30f1754a