Fix buildworld by adding DPADD= to libdpv [dpv(3)] Makefile
NB: Should also address `make -j' building
Remove "+" from "+=" in assignments to DPADD/LDADD while here.
NB: Also move CFLAGS for style measure.

Reviewed by:	shurd
MFC after:	21 days
X-MFC-to:	stable/10 stable/9
X-MFC-with:	274116 274120 274121