distcheck: add support for AM_DISTCHECK_CONFIGURE_FLAGS

distcheck: add support for AM_DISTCHECK_CONFIGURE_FLAGS

* doc/automake.texi (Checking the Distribution): Suggest to use
AM_DISTCHECK_CONFIGURE_FLAGS, not DISTCHECK_CONFIGURE_FLAGS, to
define (in the top-level Makefile.am) extra flags to be passed
to configure at "make distcheck" time; DISTCHECK_CONFIGURE_FLAGS
should be reserved for the user.  Add proper `@vindex' directive.
Document that AM_DISTCHECK_CONFIGURE_FLAGS is not honoured in a
subpackage Makefile.am, but the flags in it are passed down to
the configure script of the subpackage.
* lib/am/distdir.am (distcheck): Also pass the flags in
$(AM_DISTCHECK_CONFIGURE_FLAGS) to the configure invocation.
Update comments.
* tests/defs.in.test (AM_DISTCHECK_CONFIGURE_FLAGS,
DISTCHECK_CONFIGURE_FLAGS): Unset in case they are exported in
the environment, they might improperly influence our testsuite.
* tests/distcheck-configure-flags.test: New test.
* tests/distcheck-configure-flags-am.test: Likewise.
* tests/distcheck-configure-flags-subpkg.test: Likewise.
* distcheck-hook.test: Likewise.
* distcheck-hook2.test: Likewise.
* tests/Makefile.am (TESTS): Update.

Closes automake bug#8487.