Do a custom build of podd documentation

  If the podd docs are built using the ROOT tagfile, then when the hcana
  docs are built using the podd tag file, doxygen crashes.  This commit
  adds building of the podd docs to the hcana doc building Makefile.  The
  podd docs are built without using the ROOT tagfile, so a sed hack is used
  to modify podd's Doxyfile.