Per file options

[hsyl20]

To build GHC's RTS, Hadrian uses some feature that isn't available in cabal (afaik): setting custom cc-options or cpp-options per file/glob.

See https://gitlab.haskell.org/ghc/ghc/-/blob/8f2e08320f846ed72ef0306ebbe75bdb3752226a/hadrian/src/Settings/Packages.hs#L403:

          -- We're after pure performance here. So make sure fast math and
          -- vectorization is enabled.
          , input "**/Hash.c" ? pure [ "-O3" ]

          , inputs ["**/Evac.c", "**/Evac_thr.c"] ? arg "-funroll-loops"

          , speedHack ?
            inputs [ "**/Evac.c", "**/Evac_thr.c"
                   , "**/Scav.c", "**/Scav_thr.c"
                   , "**/Compact.c", "**/GC.c" ] ? arg "-fno-PIC"

It would be nice to convert this into proper Cabal entries so that we get one step closer to build the boot libraries with cabal. Something like:

    cc-options-custom:
        <glob> <options>
    cpp-options-custom:
        <glob> <options>
    ghc-options-custom:
        <glob> <options>

E.g.

    cc-options-custom:
        **/Compact.c -finline-limit=2500
        **/Hash.c    -O3
        {**/Evac.c,**/Evac_thr.c} -funroll-loops

    if flag(speedHack)
      cc-options-custom:
        **/{Evac.c, Evac_thr.c, Scav.c, Scav_thr.c, Compact.c, GC.c} -fno-PIC

    cpp-options-custom:
        **/RtsUtils.c -DProjectVersion=foo -DHostPlatform=bar ...

[hsyl20]

@andreabedini has a patch to allow options to be specified after source paths. It's simpler than my proposal to add new sections with path globing. Could you open a PR with your patch @andreabedini?