Some other problem that occured to me is that precompilation in 7.8 does not work with a recent 7.9 because it requires the API version to be specified (version 3) on the command line. Maybe its enough to simply backport grubbas api version feature, or better make 7.9 precompile compatible with 7.8.
Do you have a list of the files that fail to precompile with 7.9?
The api feature is intentionally strict, since precompiling with the wrong api version may cause strange errors that are first noticed when the resulting binary is used.
/grubba