install: pkglibexec_SCRIPTS is a valid prefix/primary combination

install: pkglibexec_SCRIPTS is a valid prefix/primary combination

It makes little sense for `libexec_SCRIPTS' to be accepted as valid
while `pkglibexec_SCRIPTS' is not.  So fix this inconsistency by
explicitly allowing `pkglibexec_SCRIPTS' as well.  It is worth
noting that the inconsistency has been there for a long time,
but only the quite recent commit `v1.11-373-g9ca6326' "Warnings
about primary/prefix mismatch fixed and extended" has made it
noisy enough to be noticed.

* automake.in (handle_scripts): Also list `pkglibexec' among the
prefixes valid for the `SCRIPTS' primary.
* doc/automake.texi (Scripts): Likewise.
* tests/primary-prefix-valid-couples.test: Update.
* THANKS: Likewise.
* NEWS: Likewise.

Reported by Dennis Schridde on the automake list:
<http://lists.gnu.org/archive/html/automake/2012-01/msg00002.html>