info: allow user to inhibit creation/update of '${infodir}/dir'

info: allow user to inhibit creation/update of '${infodir}/dir'

With this change, we allow the user to request the install-info
rules not to update the `${infodir}/dir' file, by exporting the
environment variable `AM_UPDATE_INFO_DIR' to the value "no".
This is especially useful to distro packagers, and is a definite
improvement over our previous hack of looking whether the
`install-info' program was the Debian or GNU version -- hack
which had been silently broken with recent versions of debian
install-info BTW (probably since dpkg 1.15.4, 2009-09-06).
This change fixes automake bug#9773.  See also Debian Bug#543992.

* lib/am/texinfos.am: Don't look anymore at the output of
`install-info --version' to decide whether to use it to update
the `${infodir}/dir' or not; instead, honour the environment
variable `AM_UPDATE_INFO_DIR'.
* tests/install-info-dir.test: New test.
* tests/Makefile.am (TESTS): Add it.
* tests/defs: Also unset `AM_UPDATE_INFO_DIR', to avoid unwanted
interferences from the environment.
* doc/automake.texi (Texinfo): Update.
* NEWS: Likewise.
* THANKS: Likewise.
Report by Jonathan Nieder.