libtool.m4: Cleanup sysroot trailing "/"

If $CC has --sysroot=/, it is a valid configuration however libtool will
then set lt_sysroot to "/".

This means references like $lt_sysroot$libdir become //usr/lib instead
of the more normally expected /usr/lib. This may or may not break something
but certainly is confusing to the user and gives confusing output. Making
"/" simply unset lt_sysroot is much cleaner.

Whilst here, trim any trailing '/' from sysroot paths to drop the duplication
and result in cleaner/consistent output.

* m4/libtool.m4: Cleanup sysroot trailing '/' handling

Upstream-Status: Backport [https://git.savannah.gnu.org/cgit/libtool.git/commit/?id=365805327c7b9bbdb0e622b954b6b0d8eaeb3f99]
Signed-off-by:  Richard Purdie <richard.purdie@linuxfoundation.org>

---
 m4/libtool.m4 | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/m4/libtool.m4 b/m4/libtool.m4
index fa1ae91..2f31d24 100644
--- a/m4/libtool.m4
+++ b/m4/libtool.m4
@@ -1256,7 +1256,9 @@ lt_sysroot=
 case $with_libtool_sysroot in #(
  yes)
    if test yes = "$GCC"; then
-     lt_sysroot=`$CC --print-sysroot 2>/dev/null`
+     # Trim trailing / since we'll always append absolute paths and we want
+     # to avoid //, if only for less confusing output for the user.
+     lt_sysroot=`$CC --print-sysroot 2>/dev/null | $SED 's:/\+$::'`
    fi
    ;; #(
  /*)