[PATCH] dist: add new "pure-dist" automake option

Since v1.15.1-204-gac47c22e3, "make dist" has been depending on
$(BUILT_SOURCES) to avoid problems when building some GNU packages which
need to compile themselves as part of building their tarballs (for
example, to generate manpage content from --help output).  This default
behaviour might be "too much" for other projects, though, so add a new
option "pure-dist" to disable it.

* NEWS:
* bin/automake.in:
* doc/automake.texi: Add a new option "pure-dist" to disable the
dependency of distdir: on $(BUILT_SOURCES).
* t/pure-dist.sh:
* t/impure-dist.sh: Add a pair of tests for a similar scenario with and
  without the option.

Fixes automake bug https://debbugs.gnu.org/49317
---
 NEWS                    |  3 ++
 bin/automake.in         |  3 +-
 doc/automake.texi       |  7 +++++
 lib/Automake/Options.pm |  1 +
 lib/am/distdir.am       |  5 ++++
 t/impure-dist.sh        |  1 +
 t/pure-dist.sh          | 66 +++++++++++++++++++++++++++++++++++++++++
 7 files changed, 85 insertions(+), 1 deletion(-)
 create mode 120000 t/impure-dist.sh
 create mode 100644 t/pure-dist.sh