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>

 2008-06-07  Mike Frysinger  <vapier@gentoo.org>

 	* sysdeps/unix/sysv/linux/alpha/sys/user.h: Copy page.h definitions

 	from the kernel header to sys/user.h and remove the #include of

 	sys/user.h.

 --- a/sysdeps/unix/sysv/linux/alpha/sys/user.h

 +++ b/sysdeps/unix/sysv/linux/alpha/sys/user.h

 @@ -23,7 +23,6 @@

     only. Don't read too much into it. Don't use it for anything other

     than gdb/strace unless you know what you are doing. */

 -#include <asm/page.h>

  #include <asm/reg.h>

  struct user

 @@ -41,6 +40,9 @@ struct user

    char u_comm[32];				/* user command name */

  };

 +#define PAGE_SHIFT		13

 +#define PAGE_SIZE		(1 << PAGE_SHIFT)

 +#define PAGE_MASK		(~(PAGE_SIZE-1))

  #define NBPG			PAGE_SIZE

  #define UPAGES			1

  #define HOST_TEXT_START_ADDR	(u.start_code)