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.
Put debug files in GDB's debug-file-directory

This puts the .debug file in the separate root reserved for such, as
pointed to by GDB's debug-file-directory setting, see
<http://sourceware.org/gdb/onlinedocs/gdb/Separate-Debug-Files.html>

Notice that this is *outside* of the prefix tree!