cabal-version: 3.4 Don't generate `Paths_pkgname.hs` when it's not in `autogen-modules`

[phadej]

This is following explicit rule of "don't do anything you are not told to do".

• cabal-version: 3.4. only generate Paths_pkgname.hs if there is autogen-modules: Paths_pkgname (note, other-modules would not be enough!)
• cabal-version: 3.0 or older: generate Paths_pkgname.hs always.

Along the same vein we could not generate cabal_macros.h (if there's no *-extensions: CPP), but that would be a silent breakage as recent GHCs provide some of definitions. So for cabal_macros.h, the best we can do is to not generate additional TOOL_* macros. #6442 is related

• cabal-version: 3.4 and recent GHC: generate cabal_macros.h only if there is *-extensions: CPP
• cabal-versions: 3.4 and old GHC:
  • generate full cabal_macros.h if there's *-extensions: CPP
  • generate only TOOL_ and CURRENT_ definitions if there is not *-extensions: CPP
• cabal-version: 3.0 or older, and recent GHC: generate only TOOL_ and CURRENT_ definitions
• cabal-version: 3.0 or older, and old GHC: generate full cabal_macros.h

Note: currently Paths_pkgname.hs uses CPP itself, but it can be changed.
We can generate Paths file suitable for specific compiler/dependencies version.

Discussion time: until 2020-02-01