support DESTDIR install

--- bash-2.05b/Makefile.in
+++ bash-2.05b/Makefile.in
@@ -20,16 +20,16 @@
 all: .made
 
 # Include some boilerplate Gnu makefile definitions.
-prefix = @prefix@
+prefix = $(DESTDIR)@prefix@
 
-exec_prefix = @exec_prefix@
-bindir = @bindir@
-libdir = @libdir@
-infodir = @infodir@
-includedir = @includedir@
+exec_prefix = $(DESTDIR)@exec_prefix@
+bindir = $(DESTDIR)@bindir@
+libdir = $(DESTDIR)@libdir@
+infodir = $(DESTDIR)@infodir@
+includedir = $(DESTDIR)@includedir@
 datadir = @datadir@
 
-mandir = @mandir@
+mandir = $(DESTDIR)@mandir@
 manpfx = man
 
 man1ext = .1