bsd.prog.mk: Avoid redefining $PROG target

In rare cases, the PROG target is intentionally redefined before
including bsd.prog.mk, but when that happens a target redefinition
warning is emitting.  This adds a check for a pre-existing $PROG
target before attempting to define it.

It stops warnings from being emitted when loader and loader_tftp
are built.

share/mk/bsd.prog.mk

@@ -32,12 +32,14 @@ LDADD+=	${OBJCLIBS}
 
 OBJS+=  ${SRCS:N*.h:N*.patch:R:S/$/.o/g}
 
+.if !target(${PROG})
 ${PROG}: ${OBJS}
 .if defined(PROG_CXX)
 	${CXX_LINK} ${CXXFLAGS} ${LDFLAGS} -o ${.TARGET} ${OBJS} ${LDADD}
 .else
 	${CC_LINK} ${CFLAGS} ${LDFLAGS} -o ${.TARGET} ${OBJS} ${LDADD}
 .endif
+.endif
 
 .else # !defined(SRCS)