Misc. Doxygen configuration changes

Misc. Doxygen configuration changes

- Adjust Doxygen preprocessor behavior such that it actually parses
  #ifdefs in the code.  Otherwise, some parts of the SIMD code seemed to
  get ignored.
- Extract static functions on all levels of documentation for
  consistency (and we may end up with some static inline members in
  public headers; currently at least vec.h qualifies).
- Remove unnecessary excludes now that VERBATIM_HEADERS=NO already
  excludes the unwanted files.
- Some other minor cleanup.
- Move member groups higher up in the layouts, since they generally are
  the most interesting part.

Change-Id: Iea04772628716ed12f44cd4465c2027f4670bfae