more m4_pattern_forbid issues

[Ralf Wildenhues]

This could be related to Noah's proposed patch, but the error happens
(with slightly different output) with or without the patch.

cat >configure.ac <<EOF
AC_INIT
AS_FOO
EOF
cat >aclocal.m4 <<EOF
m4_define([AS_FOO], [
  AS_GOO
])
EOF
rm -rf autom4te.cache
autoconf

produces the expected output:
| aclocal.m4:2: error: possibly undefined macro: AS_GOO
|       If this token and others are legitimate, please use m4_pattern_allow.
|       See the Autoconf documentation.

Subsequent `autoconf' invocations add more and more instances of this
warning:
| Use of uninitialized value in join or string at
/home/ralf/local/share/autoconf/Autom4te/C4che.pm line 73, <GEN1> line 181.

presumably due to entries like this one in autom4te.cache/requests: | 
bless( [
|           '4',
|           1,
|           [
|             '/home/ralf/local/share/autoconf'
|           ],
|           [
|             '/home/ralf/local/share/autoconf/autoconf/autoconf.m4f',
                    'AS_GOO
|
|             undef
|           ],
|           {
|             '_LT_AC_TAGCONFIG' => 1,
[...]

Where does that `undef' come from?

Cheers,
Ralf