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>

 Enable using a shell other than bash.

 Courtesy "Martin Guy" <martinwguy@yahoo.it>

 http://sourceware.org/ml/crossgcc/2008-08/msg00037.html

 --- binutils-2.18.orig/ld/genscripts.sh	2007-08-12 20:00:07.000000000 +0100

 +++ binutils-2.18/ld/genscripts.sh	2008-08-25 16:09:54.000000000 +0100

 @@ -398,9 +398,11 @@

      . $em_script

      em_script=$current_script

    }

 -  fragment()

 +  

 +  alias fragment='em_lineno=$LINENO; fragment_em'

 +  fragment_em()

    {

 -    local lineno=$[${BASH_LINENO[0]} + 1]

 +    local lineno=$[$em_lineno + 1]

      echo >> e${EMULATION_NAME}.c "#line $lineno \"$em_script\""

      cat >> e${EMULATION_NAME}.c

    }