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>

 --- glibc-2.6.1/sysdeps/unix/sysv/linux/kernel-features.h	2006-07-29 07:43:27.000000000 +0300

 +++ glibc-2.6.1/sysdeps/unix/sysv/linux/kernel-features.h.new	2008-07-26 08:45:06.000000000 +0300

 @@ -445,7 +445,7 @@

  /* The *at syscalls were introduced just after 2.6.16-rc1.  Due to the way the

     kernel versions are advertised we can only rely on 2.6.17 to have

     the code.  */

 -#if __LINUX_KERNEL_VERSION >= 0x020611

 +#if __LINUX_KERNEL_VERSION >= 0x020611 && !defined __alpha__

  # define __ASSUME_ATFCTS	1

  #endif