Make the build system respect DESTDIR, in order to allow staged builds.

--- a/Makefile.in
+++ b/Makefile.in
@@ -109,13 +109,13 @@
 #          installs man pages in MANDIR/man1/  (e.g. if MANSUFFIX is 1)
 #          Creates these directories if they don't exist.
 install: 
-	mkdir -p ${BINDIR}
-	-mkdir -p ${MANDIR}/man${MANSUFFIX}
+	mkdir -p ${DESTDIR}${BINDIR}
+	-mkdir -p ${DESTDIR}${MANDIR}/man${MANSUFFIX}
 	for file in $(PROGS) $(PVMPROGS); do\
-	   cp src/$$file $(BINDIR)/;\
+	   cp src/$$file ${DESTDIR}$(BINDIR)/;\
 	done
 	-for file in hmmer $(PROGS); do\
-	   $(INSTMAN) documentation/man/$$file.man $(MANDIR)/man$(MANSUFFIX)/$$file.$(MANSUFFIX);\
+	   $(INSTMAN) documentation/man/$$file.man ${DESTDIR}$(MANDIR)/man$(MANSUFFIX)/$$file.$(MANSUFFIX);\
 	done
 
 # uninstall: Reverses the steps of "make install".