Fix response file handling.

[samhocevar]

Currently the response file handling for UE4.13 has two problems:

• it lists the .obj files as prebuild dependencies instead of inputs, so if the response file is not modified between builds, UE4Game.exe will be considered up to date even if some .obj were regenerated.
• it uses the Copy() node which — by design — does not update the timestamp of its output, so if the response file is not modified between builds, the subsequent targets are always considered up to date.

Unfortunately FASTBuild does not accept object lists as an action’s inputs, so this change will require applying the following patch to FASTBuild: https://github.com/fastbuild/fastbuild/pull/149

[liamkf]

Hmmmmmm yeah the Copy step is definitely a bodge, as I'm sure you've seen... :) if the exec and lib nodes supported the dependencies I'd love to get rid of it.

What you've done seems very reasonable as well!

If we wanted to avoid modifying FASTBuild could just calling something like File.SetLastWriteTimeUtc(ResponseFile, DateTime.UtcNow) on any response files we see for linker actions work? Then the Copy node would know it needs to run. I haven't spent much time on the latest with the response file silliness unfortunately...