* lt~obsolete.m4: Update to libtool 2.2.6.
* ltmain.sh: Update to libtool 2.2.6.
* ltsugar.m4: Update to libtool 2.2.6.
* ltversion.m4: Update to libtool 2.2.6.
* ltoptions.m4: Update to libtool 2.2.6.
* ltgcc.m4: Update to match changes from libtool 2.2.6.