doc/Doxyfile.in: update to recent version compatibility
doc/Doxyfile.in: update to recent version compatibility

Update to doxygen 1.8.x, also enable creation of DOT graphics.

gebart/libcoap,authmillenon/libcoap,authmillenon/libcoap,gebart/libcoap,authmillenon/libcoap,gebart/libcoap