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.

 -= BEGIN original header =-

 -= END original header =-

 diff -durN uClibc-0.9.30.orig/Rules.mak uClibc-0.9.30/Rules.mak

 --- uClibc-0.9.30.orig/Rules.mak	2008-11-12 13:24:16.000000000 +0100

 +++ uClibc-0.9.30/Rules.mak	2009-02-07 09:57:59.000000000 +0100

 @@ -519,6 +519,7 @@

  endif

  CFLAGS += $(call check_gcc,-std=gnu99,)

 +CFLAGS += $(call check_gcc,-fgnu89-inline,)

  LDFLAGS_NOSTRIP:=$(CPU_LDFLAGS-y) -shared \

  	-Wl,--warn-common -Wl,--warn-once -Wl,-z,combreloc