Move `unix/prebuild.sh` input files to dedicated subdirectory.

[c-lipka]

Cleared the unix/ folder of any files to be processed by unix/prebuild.sh, and moved the respective files to the unix/prebuild/ subfolder instead. Renamed install.txt back to its canonical name INSTALL in the process, as this also solves the potential name conflict with the unix/install script on case-insensitive file systems.