message: build: don't warn when doxygen isn't found
  Doxygen isn't so important that we need to warn when it is missing. I'd
  assume it might even be missing more often than not for most builds.
diff: @@ -119,9 +119,6 @@ AC_PATH_TOOL(READELF, readelf)
  AC_PATH_TOOL(CPPFILT, c++filt)
  AC_PATH_TOOL(OBJCOPY, objcopy)
  AC_PATH_PROG(DOXYGEN, doxygen)
  -if test -z "$DOXYGEN"; then
  - AC_MSG_WARN([Doxygen not found])
  -fi
  AM_CONDITIONAL([HAVE_DOXYGEN], [test -n "$DOXYGEN"])
  AC_ARG_VAR(PYTHONPATH, Augments the default search path for python module files)
  