Having CmakeListsCustom.txt in git defeats the purpose of its existence:
If it is used, there is a modified file in git.
Fix this by only including the file if it exists.
Other changes:
- Rename the existing Custom files to CMakeListsCustom.txt.example.
- Move the plugins custom file to the top level (same level as its
  including parent).
- Optionally allow a list of custom includes instead of the default one.

Change-Id: I8960eac6222f741c045055d43d1d5a2d4979caf6
Reviewed-on: https://code.wireshark.org/review/4163
Reviewed-by: Jörg Mayer <jmayer@loplof.de>