libtool.m4: Cleanup sysroot trailing "/"

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.