1. Update for ROOT5-enabled root_generate_dictionary CMake function

  root_generate_dictionary now works with both ROOT 5 and ROOT 6.
  It defines the dictionary custom target and PCM file internally,
  as appropriate, and also automatically extracts any required
  -D compile definitions from the given target(s).

2. Add configuration option HCANA_BUILTIN_PODD (default ON)

  If set to OFF (cmake -DHCANA_BUILTIN_PODD=OFF ...), CMake will find a
  pre-built version of Podd according to the usual find_package search
  rules. (It's sufficient to have the installed "analyzer" in your PATH,
  but you can also set CMAKE_PREFIX_PATH to the Podd installation root,
  for example.) This version of Podd has to have been built and
  installed with Podd's CMake system. In this case, only libHallC will
  be built, and the Podd submodule of hcana is ignored and does not even
  have to be present.

3. Clean existing src/hc_compiledata.h during CMake configuration

  This file might be left over from a previous non-CMake build and would
  be incorrectly picked up instead of the CMake-generated version.

4. Generate hcanaConfig.cmake config and version files using latest Podd scripts

  Requires Podd at commit JeffersonLab/analyzer@34dd368