remove sbase-box from PHONY

I have noticed this issue while I was trying to package sbase-box. Since
sbase-box was in the PHONY target, running sbase-box-install caused it to
be rebuilt when it was unnecessary. This also caused options passed to make
to be lost (such as LDFLAGS).

Makefile

@@ -279,4 +279,4 @@ clean:
 .gitignore:
 	{ printf '*.o\n' ; printf '/%s\n' getconf.h $(LIB) $(BIN) ; } > $@
 
-.PHONY: all install uninstall dist sbase-box sbase-box-install sbase-box-uninstall clean .gitignore
+.PHONY: all install uninstall dist sbase-box-install sbase-box-uninstall clean .gitignore