nested [] with whitespace in misc description leads to _m4_text_wrap_word errors

[Mike Frysinger]

some packages like to use [] in their desc and rather than use quadrigraphs, 
they throw more [] at the problem.  for example:
AC_ARG_ENABLE(exa, AC_HELP_STRING([--disable-exa],
 [Disable EXA support [[default enabled]]]),
 [EXA="$enableval"], [EXA=yes])

or this:
AC_ARG_VAR(PERLLD, [[same as PERLCC] Linker for Perl modules])

the exact output actually generated seems to vary widely over time (looking at 
2.59, 2.61, 2.63, 2.64).  however, while versions before 2.64 would at least 
output something, 2.64 simply aborts with an unhelpful message:

$ cat configure.ac
AC_INIT
AC_ARG_VAR(VAR, [[cow moo] desc])
AC_OUTPUT

$ autoconf
configure:1210: error: possibly undefined macro: _m4_text_wrap_word
      If this token and others are legitimate, please use m4_pattern_allow.
      See the Autoconf documentation.

people have no idea where/why this error is coming from (although looking at 
the generated configure does yield some pointers).

if there is no white space between the [], then things actually work and the 
output is what people expect:
AC_INIT
AC_ARG_VAR(VAR, [[cow-moo] desc])
AC_OUTPUT
$ autoconf && ./configure --help | grep cow-moo
  VAR         [cow-moo] desc
-mike