Log Message:
graphics/graphviz: fix build with BSD make

BSD make does not know the $< variable in explicit rules, it only knows
it for inference rules like ".c.o".  This resulted in $< expanding to an
empty string, which in turn got the build stuck because it tried to read
from stdin.  But not in parallel mode, where stdin was redirected to an
empty file, resulting in an empty PostScript file being installed.

Fixes PR pkg/55539.