ENH: Introduce "rule hashes" to help rebuild files when rules change.

  - In CMake 2.4 custom commands would not rebuild when rules changed.
  - In CMake 2.6.0 custom commands have a dependency on build.make
    which causes them to rebuild when changed, but also when any
    source is added or removed.  This is too often.
  - We cannot have a per-rule file because Windows filesystems
    do not deal well with lots of small files.
  - Instead we add a persistent CMakeFiles/CMakeRuleHashes.txt file
    at the top of the build tree that is updated during each
    CMake Generate step.  It records a hash of the build rule for
    each file to be built.  When the hash changes the file is
    removed so that it will be rebuilt.

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