Only run Doxygen when necessary.
Rename DX_SOURCES to DX_SRCS.

This so we don't confuse Automake (since _SOURCES is a special suffix).
Run Doxygen in quiet mode.
Add output when silent-rules is in effect.

When using "make V=0" to enable silent-rules, it will now show:
  GEN  ./sdl_pangodraw.tag

The name is not optimal, but it's better than nothing.