init: ensure $ac_aux_dir is defined before being used

init: ensure $ac_aux_dir is defined before being used

Since we use '$ac_aux_dir' to define '$am_aux_dir', we need
to ensure the former has been initialized before we try to
define the latter, otherwise the definition:

   am_aux_dir=`cd $ac_aux_dir && pwd`

will set '$am_aux_dir' to '$HOME', likely causing weird and
unexpected behaviours.

This change fixes automake bug#15981.

* m4/auxdir.m4 (AM_AUX_DIR_EXPAND): AC_REQUIRE expansion
of 'AC_CONFIG_AUX_DIR_DEFAULT'. Fix redundant comment and
AC_PREREQ, add extra quoting around '$ac_aux_dir'.
* t/auxdir-pr15981.sh: New test.
* t/auxdir-cc-pr15981.sh: Likewise.
* t/list-of-tests.mk (handwritten_TESTS): Add them.
* THANKS, NEWS: Update.
* bin/automake.in: Fix a harmless typo in comments, that
I happened to notice while writing this patch.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>