Issue information about --prefix and DESTDIR in help

Use --prefix= in ./configure to enter the directory under which the
files should be installed on the target system, use DESTDIR= in
make install to redirect files to another location when building.

configure

@@ -5,7 +5,8 @@ usage () {
   cat <<EOF
 Installation directories:
   --prefix=PREFIX         install architecture-independent files in PREFIX
-                          [/usr/local]
+                          [/usr/local]. Note: set DESTDIR=PATH when doing
+                          \`make install' to install to a different sysroot.
 
 Optional Features:
   --disable-FEATURE       do not include FEATURE