bug#19311: regression: 1.14 may use ac_aux_dir before defined (again)

[Jan Engelhardt]

When AC_SYSTEM_EXTENSIONS precedes AM_INIT_AUTOMAKE, it used to throw an 
error - which has been fixed in
http://debbugs.gnu.org/cgi/bugreport.cgi?bug=15981 .

I have here another instance of what appears to be a similar issue. This 
one is also a regression (used to work in 1.13.4), and the regression is 
still present in {automake 1.14.1 + patch from #15981}. The problem 
originates in libmemcached/configure.ac, and I have produced the 
following reduced testcase that exhibits the issue:

$ cat <<EOF >configure.ac
AC_INIT([foo], [0])
AC_PROG_CC([cc gcc clang])
AC_CONFIG_AUX_DIR([build-aux])
AC_CONFIG_MACRO_DIR([m4])
AM_INIT_AUTOMAKE([1.11 subdir-objects foreign])
AC_OUTPUT([Makefile])
EOF
$ echo '' >Makefile.am
$ md m4
$ autoreconf -fi
configure.ac:2: installing 'build-aux/compile'
configure.ac:5: installing 'build-aux/install-sh'
configure.ac:5: installing 'build-aux/missing'
$ ./configure
checking for cc... cc
checking whether the C compiler works... yes
checking for C compiler default output file name... a.out
checking for suffix of executables... 
checking whether we are cross compiling... no
checking for suffix of object files... o
checking whether we are using the GNU C compiler... yes
checking whether cc accepts -g... yes
checking for cc option to accept ISO C89... none needed
configure: error: cannot find install-sh, install.sh, or shtool in "." "./.."
"./../.."

With automake-1.13.4, configure would succeed through to the end 
("config.status: creating Makefile").