[commit: ghc] wip/erikd/rts's head updated: rts: Add `-Wundef` to CFLAGS and fix warnings (e57c199)

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Repository : ssh://git@git.haskell.org/ghc

Branch 'wip/erikd/rts' now includes:

     1e67010 RtsFlags.c: Const correct fixes
     fd0d0fc Fix detection and use of `USE_LIBDW`
     e57c199 rts: Add `-Wundef` to CFLAGS and fix warnings