Why does ext/zlib have config0.m4 instead of config.m4

On Mac OS X, to build the zlib.so shared module, I can run PHP's main  
"configure" script with the "--with-zlib=shared,[DIR]" argument and  
then run "make build-modules".

Most of what PHP's main configure script does is irrelevant to  
building just one extension, and for most extensions, I can make this  
step go faster by not calling PHP's main configure script, but  
instead running "phpize", "./configure" and "make" in the extension  
directory itself. For the zlib extension, though, phpize complains  
that there isn't a file called config.m4. There is a file called  
config0.m4, though, and renaming that to config.m4 seems to make it  
work properly.

Somebody else noticed this in December 2002 and reported it as a bug:

http://bugs.php.net/bug.php?id=21171

It was closed as bogus, with the note "dont try to phpize the zlib  
extension" and "Work is in progress to
disallow building zlib as an shared extension".

Since that was almost seven years ago and today's PHP 5.3.0 still has  
no problem building zlib as a shared extension through its main  
configure script, are the statements in that bug note accurate? If  
not, why does zlib have config0.m4 instead of config.m4?