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/2.19/91_all_libiberty-pic.patch

 diff -durN binutils-2.19.orig/libiberty/Makefile.in binutils-2.19/libiberty/Makefile.in

 --- binutils-2.19.orig/libiberty/Makefile.in	2008-07-24 15:51:49.000000000 +0200

 +++ binutils-2.19/libiberty/Makefile.in	2008-11-23 17:03:33.000000000 +0100

 @@ -227,6 +227,7 @@

  	  $(AR) $(AR_FLAGS) $(TARGETLIB) \

  	    $(REQUIRED_OFILES) $(EXTRA_OFILES) $(LIBOBJS); \

  	  $(RANLIB) $(TARGETLIB); \

 +	  cp $(TARGETLIB) ../ ; \

  	  cd ..; \

  	else true; fi