Makefile: speedup dependency generation

Dependencies can be generated in parallel which can be much faster. It
also puts away the problem that for may fail multiple times in a row and
leaving behind a broken intermediate file which would be picked up by
successive runs.

Signed-off-by: Kai Krakow <kai@kaishome.de>

.gitignore

@@ -1,6 +1,7 @@
 *.[ao]
 *.bak
 *.new
+*.dep
 *.so*
 Doxyfile
 README.html