libtool: fix resolve of lt_sysroot

libtool: fix resolve of lt_sysroot

This patch updates libtool.m4 (and its output) to resolve a problem
with variable 'lt_sysroot' not being properly updated if the option
'--with[-libtool]-sysroot' is not provided when running the 'configure'
script for a package.

According to the help text ouput from 'configure':
--with-libtool-sysroot=DIR Search for dependent libraries within DIR
                        (or the compiler's sysrooot if not specified).

Due to mixed up cases in a switch statement, when checking if the option
was specified or not, wrong actions were taken resulting in an incorrect
sysroot and failures to properly locate e.g. .la files when using the
populated SDK toolchain.

For current upstream status see:
http://lists.gnu.org/archive/html/bug-libtool/2013-09/msg00005.html

(From OE-Core rev: f5cf7e1a5c85fb320faa9cbeef24f491706b4c1d)

Signed-off-by: Hans Beckerus <hans.beckerus at gmail.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>