scripts/xldd: use user's sed and grep

xldd uses sed and grep as detected by ./configure. This works well if is
used on the machine that build the toolchain.

But if the user moves the toolchain to another machine where sed and grep
are not in the same directory (eg. /bin/sed vs. /usr/bin/sed), then xldd
will stop functionning.

Fix that by using ${SED} and ${GREP} if they are set in the environment.

Reported-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

diff -ru ltrace-0.5.3.org/configure ltrace-0.5.3/configure

--- ltrace-0.5.3.org/configure	2011-08-21 18:55:15.000000000 +0200

+++ ltrace-0.5.3/configure	2011-08-21 18:54:46.000000000 +0200

@@ -15,8 +15,12 @@

 echo $PACKAGE_VERSION

 echo -n "checking HOST_OS... "

-HOST_OS=$( uname -s )

-if [ "$HOST_OS" = "Linux" ]

+if [ -z "$HOST_OS" ] ; then

+  HOST_OS=$( uname -s )

+else

+  echo -n "using preset: "

+fi

+if [ "$HOST_OS" = "Linux" -o "$HOST_OS" = "linux" ]

 then

   HOST_OS="linux-gnu"

 fi