parallel-tests: fix another BSD parallel make issue

parallel-tests: fix another BSD parallel make issue

When BSD make is run in parallel mode, it apparently strips any
leading directory component from the automatic variable '$*' (of
course, against what POSIX mandates).  This was causing FreeBSD 9.0
make and NetBSD 5.1 make to spuriously fail with automake-generated
test harnesses if subdir tests were present *and* make was being
run in parallel mode.  This issue affected also the Automake own
testsuite.

* lib/am/check2.am (am__set_b): New internal variable.
(%OBJ%, %EXT%.log, %EXT%$(EXEEXT).log): Use it to work around
the described BSD make issue.
* tests/parallel-tests3.test: Enhanced to expose the bug.
* tests/parallel-tests-subdir.test: Enhance a little, since we
are at it.
* NEWS: Update.