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:

gentoo/src/patchsets/binutils/2.19/03_all_binutils-2.15.92.0.2-ppc64-pie.patch

diff -durN binutils-2.19.orig/bfd/elf64-ppc.c binutils-2.19/bfd/elf64-ppc.c

--- binutils-2.19.orig/bfd/elf64-ppc.c	2008-10-09 14:18:24.000000000 +0200

+++ binutils-2.19/bfd/elf64-ppc.c	2008-11-23 16:16:11.000000000 +0100

@@ -11294,7 +11294,12 @@

 	      else if (!SYMBOL_REFERENCES_LOCAL (info, &h->elf)

 		       && !is_opd

 		       && r_type != R_PPC64_TOC)

-		outrel.r_info = ELF64_R_INFO (h->elf.dynindx, r_type);

+		{

+		  outrel.r_info = ELF64_R_INFO (h->elf.dynindx, r_type);

+		  if (h->elf.dynindx == -1

+		      && h->elf.root.type == bfd_link_hash_undefweak)

+		  memset (&outrel, 0, sizeof outrel);

+		}

 	      else

 		{

 		  /* This symbol is local, or marked to become local,