Fix for PR automake/285:

Fix for PR automake/285:
* automake.in (handle_ltlibraries): Keep track of installation
directories for each condition, then define a $(am_TARGET_rpath)
variable to hold the -rpath flags of Libtool libraries conditionally
installed in different directories.
* lib/Automake/DisjConditions.pm (merge): New function.
* tests/libtool6.test: Adjust.
* tests/libtool8.test: New file.
* tests/Makefile.am (TEST): Add libtool8.test.