config: add an option not to remove the destination directory

In certain circumstances, removing the destination/installation directory
is a bad idea. For example, when the build environment is already taking
care of sanitising the build tree, and pre-installs stuff in there, it is
a very bad idea to remove the destination directory.

This happens now in buildroot, as the crostool-NG backend now installs the
toolchain in the common host-tools directory, and pre-install there a few
host-utilities (eg. host-automake and host-gawk).

Provide a config knob to turn on/off the removal of the destination
directory, defaulting to 'y' (previous behavior), and forced to 'n' when
used as a backend.

Reported-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

Original patch from: gentoo/src/patchsets/glibc/2.9/6017_all_alpha-glibc-2.8-creat.patch

-= BEGIN original header =-

alpha does not have a __NR_creat

http://bugs.gentoo.org/227275

http://sourceware.org/bugzilla/show_bug.cgi?id=6650

-= END original header =-

diff -durN glibc-2_9.orig/sysdeps/unix/sysv/linux/wordsize-64/creat64.c glibc-2_9/sysdeps/unix/sysv/linux/wordsize-64/creat64.c

--- glibc-2_9.orig/sysdeps/unix/sysv/linux/wordsize-64/creat64.c	2007-11-10 20:34:26.000000000 +0100

+++ glibc-2_9/sysdeps/unix/sysv/linux/wordsize-64/creat64.c	2009-02-02 22:01:35.000000000 +0100

@@ -1 +1,5 @@

 /* Defined as alias for the syscall.  */

+#include <sysdep.h>

+#ifndef __NR_creat

+#include "../../../../../io/creat64.c"

+#endif