message: Exclude .cpp files from Doxygen.
  Before this commit Doxygen processed all .hpp and all .cpp files.
  This resulted in duplicate documentation for functions that have
  Doxygen comments in the header and implementation in the .cpp.
  This change removes the duplicates.
diff: @@ -829,7 +829,6 @@ INPUT_ENCODING = UTF-8
  FILE_PATTERNS = *.md \
  *.markdown \
  - *.cpp \
  *.hpp \
  *.dox
  