Fix reference to debugging-related README file in README and Makefile.dj
(Apply commit 070b30d from 'master' branch.)

* doc/README: Change README.debugging to debugging.html.
* Makefile.dj (OTHER_FILES): Likewise.

Conflicts:

    Makefile.dj
Fix tests makefile to link with new GNU ld linking rules
(Apply commit 99f98bd from 'master' branch.)

* tests/tests.am (gctest_LDADD, threadleaktest_LDADD,
threadkey_test_LDADD,
initsecondarythread_LDADD): Link to pthreads (or appropriate
threading libraries) as needed; fixes failure to link with new GNU ld
linking rules.

Conflicts:

    tests/tests.am
Update (regenerate) configure files; update ChangeLog

* Makefile.in: Regenerate.