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>

 diff -dur strace-4.5.16.orig/process.c strace-4.5.16/process.c

 --- strace-4.5.16.orig/process.c	2007-01-11 23:08:38.000000000 +0100

 +++ strace-4.5.16/process.c	2007-07-14 19:19:58.000000000 +0200

 @@ -2685,7 +2685,6 @@

         { 4*REG_GBR,            "4*REG_GBR"                             },

         { 4*REG_MACH,           "4*REG_MACH"                            },

         { 4*REG_MACL,           "4*REG_MACL"                            },

 -       { 4*REG_SYSCALL,        "4*REG_SYSCALL"                         },

         { 4*REG_FPUL,           "4*REG_FPUL"                            },

         { 4*REG_FPREG0,         "4*REG_FPREG0"                          },

         { 4*(REG_FPREG0+1),     "4*REG_FPREG1"                          },