Manpages go in share/man.

Add support for DESTDIR.
--- Makefile.in.orig	2010-12-19 12:21:17.000000000 -0600
+++ Makefile.in	2020-11-12 10:24:12.000000000 -0600
@@ -4,7 +4,7 @@
 prefix=@prefix@
 exec_prefix=@exec_prefix@
 INSTALL_BIN=$(exec_prefix)/bin
-INSTALL_MAN=$(prefix)/man
+INSTALL_MAN=$(prefix)/share/man
 
 LIBS=@LIBS@
 CC=@CC@
@@ -31,10 +31,10 @@
 man: rzip64.1 
 
 install: all
-	-mkdir -p ${INSTALL_BIN}
-	${INSTALLCMD} -m 755 rzip64 ${INSTALL_BIN}
-	-mkdir -p ${INSTALL_MAN}/man1
-	${INSTALLCMD} -m 644 $(srcdir)/rzip64.1 ${INSTALL_MAN}/man1/
+	-mkdir -p $(DESTDIR)$(INSTALL_BIN)
+	$(INSTALLCMD) -m 755 rzip64 $(DESTDIR)$(INSTALL_BIN)
+	-mkdir -p $(DESTDIR)$(INSTALL_MAN)/man1
+	$(INSTALLCMD) -m 644 $(srcdir)/rzip64.1 $(DESTDIR)$(INSTALL_MAN)/man1/
 
 rzip64: $(OBJS)
 	$(CC) $(CFLAGS) -o rzip64 $(OBJS) $(LIBS)