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-2.3.6/linuxthreads/sysdeps/unix/sysv/linux/alpha/sysdep-cancel.h.orig	2008-06-19 15:31:45.000000000 +0300

+++ glibc-2.3.6/linuxthreads/sysdeps/unix/sysv/linux/alpha/sysdep-cancel.h	2008-06-19 15:36:10.000000000 +0300

@@ -54,6 +54,7 @@

 	bne	a3, SYSCALL_ERROR_LABEL;			\

 __LABEL($pseudo_ret)						\

 	.subsection 2;						\

+	cfi_startproc;						\

 __LABEL($pseudo_cancel)						\

 	subq	sp, 64, sp;					\

 	cfi_def_cfa_offset(64);					\

@@ -84,12 +85,13 @@

 	cfi_def_cfa_offset(0);					\

 __LABEL($syscall_error)						\

 	SYSCALL_ERROR_HANDLER;					\

+	cfi_endproc;						\

 	.previous

 # undef PSEUDO_END

 # define PSEUDO_END(sym)					\

-	.subsection 2;						\

 	cfi_endproc;						\

+	.subsection 2;						\

 	.size sym, .-sym

 # define SAVE_ARGS_0	/* Nothing.  */