Optimize Doxygen build

- Don't include verbatim headers in the documentation.  In addition to
  marginally reducing disk usage and processing time, this hides
  undocumented files from the file list, making it look better.
- Generate svg instead of png images. This reduces the disk usage of the
  build output by ~30%.  Change HTML file extension to .xhtml per
  Doxygen documentation to support svg images on IE.
- Add a CMake option GMX_COMPACT_DOXYGEN that disables most
  automatically generated graphs.  Compared to the original
  png-containing build, this reduces the disk usage by ~70%.

Times it takes to generate the documentation are reduced comparably.
Also enable generating multiple DOT graphs in one run, significantly
speeding up the build.  graphviz versions that don't support this are
ancient.

Change-Id: I5dd91779a8086025d9f0c0e477ce300d6a75a580