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-2.7/glibc-ports-2.7.orig/sysdeps/unix/sysv/linux/arm/nptl/sysdep-cancel.h	2007-06-06 19:27:04.000000000 +0200

 +++ glibc-2.7/glibc-ports-2.7/sysdeps/unix/sysv/linux/arm/nptl/sysdep-cancel.h	2008-05-18 16:57:21.000000000 +0200

 @@ -73,6 +73,9 @@

  # define DOCARGS_5	DOCARGS_4

  # define UNDOCARGS_5	UNDOCARGS_4

 +# define DOCARGS_6    DOCARGS_5

 +# define UNDOCARGS_6  UNDOCARGS_5

 +

  # ifdef IS_IN_libpthread

  #  define CENABLE	bl PLTJMP(__pthread_enable_asynccancel)

  #  define CDISABLE	bl PLTJMP(__pthread_disable_asynccancel)