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 H.J. Lu, as reported there:

 http://www.nabble.com/PATCH:-Use-short-for-fnstsw-td14775171.html

 fnstsw stores 16bit into %ax. The upper 16bit of %eax is unchanged.

 The new assembler (binutils-2.18.50.0.4 and up) will disallow "fnstsw %eax".

 diff -dur glibc-2.6.1.orig/sysdeps/i386/fpu/ftestexcept.c glibc-2.6.1/sysdeps/i386/fpu/ftestexcept.c

 --- glibc-2.6.1.orig/sysdeps/i386/fpu/ftestexcept.c	2004-03-05 11:14:48.000000000 +0100

 +++ glibc-2.6.1/sysdeps/i386/fpu/ftestexcept.c	2008-06-24 10:56:14.000000000 +0200

 @@ -26,7 +26,7 @@

  int

  fetestexcept (int excepts)

  {

 -  int temp;

 +  short temp;

    int xtemp = 0;

    /* Get current exceptions.  */