possible bug: no escaping for preprocessor macro STDC_HEADERS warning

Hello,

The warning for STDC_HEADERS seems to get inserted directly into the 
`configure` script, namely the following text without any quoting:

The preprocessor macro 'STDC_HEADERS' is obsolete.
   Except in unusual embedded environments, you can safely include all
   C89 headers unconditionally.)# Autoupdate added the next two lines to 
ensure that your configure

was inserted into `configure` when I ran `autoreconf -f -i` on a very old 
code base from around 2003; thus attempting to execute the commands "The" 
and "Except" and complaining of a syntax error on line of "C89".

I found the likely bug using autoconf version 2.72. However, while I am 
unfamiliar with M4, looking at lines 670 to 673 of lib/autoconf/headers.m4 
makes me think that this bug persists in the development version. Possibly 
we want to wrap the message in AC_DIAGNOSE or AC_MSG_WARN?

I considered trying my hand at writing a patch, but following the 
instructions in README-hacking and executing ./bootstrap confusingly gives 
me the error that an autoconf version 2.69 or higher is required, despite 
my system version being 2.72 and README-hacking suggesting that I 
shouldn't need autoconf installed already at all. So I think I will leave 
fixing this possible bug to people with more experience with autoconf and 
related tools.

Thank you,
     Emily Ammundsen