dot supports generating multiple graphs in one run, that was not always
the case and Doxygen had a default of not using that feature. Given it
got introduced in dot 1.8.10 and Wikimedia CI server has 2.26.x, I guess
it is safe to enable this.
End result, is less shell out overhead.
Change-Id: Ia07b430bd1af608bbe9c10eecbe2df3e0b9c74fc
DOT_GRAPH_MAX_NODES = 50
MAX_DOT_GRAPH_DEPTH = 1000
DOT_TRANSPARENT = NO
-DOT_MULTI_TARGETS = NO
+DOT_MULTI_TARGETS = YES
GENERATE_LEGEND = YES
DOT_CLEANUP = YES