backport ghc-deps.sh support for ghc-7.8 and cabal-tweak-flag manual field
    
    - ghc_fix_dynamic_rpath: abort for non-existent executable name with error msg
    - cabal-tweak-flag: add manual field to enforce flag changes
    - ghc-deps.sh fixes for bootstrapping new ghc version
      - use objdump -p instead of ldd to read executable dependencies
      - update for ghc-7.8 rts