Have the binascii module use zlib's optimized crc32() function when available

to reduce our code size (1k data table and tiny bit of code).  It falls back
to its own without zlib.

setup.py

@@ -486,9 +486,6 @@ class PyBuildExt(build_ext):
         # select(2); not on ancient System V
         exts.append( Extension('select', ['selectmodule.c']) )
 
-        # Helper module for various ascii-encoders
-        exts.append( Extension('binascii', ['binascii.c']) )
-
         # Fred Drake's interface to the Python parser
         exts.append( Extension('parser', ['parsermodule.c']) )
 
@@ -1069,6 +1066,7 @@ class PyBuildExt(build_ext):
         # You can upgrade zlib to version 1.1.4 yourself by going to
         # http://www.gzip.org/zlib/
         zlib_inc = find_file('zlib.h', [], inc_dirs)
+        have_zlib = False
         if zlib_inc is not None:
             zlib_h = zlib_inc[0] + '/zlib.h'
             version = '"0.0.0"'
@@ -1090,6 +1088,7 @@ class PyBuildExt(build_ext):
                     exts.append( Extension('zlib', ['zlibmodule.c'],
                                            libraries = ['z'],
                                            extra_link_args = zlib_extra_link_args))
+                    have_zlib = True
                 else:
                     missing.append('zlib')
             else:
@@ -1097,6 +1096,21 @@ class PyBuildExt(build_ext):
         else:
             missing.append('zlib')
 
+        # Helper module for various ascii-encoders.  Uses zlib for an optimized
+        # crc32 if we have it.  Otherwise binascii uses its own.
+        if have_zlib:
+            extra_compile_args = ['-DUSE_ZLIB_CRC32']
+            libraries = ['z']
+            extra_link_args = zlib_extra_link_args
+        else:
+            extra_compile_args = []
+            libraries = []
+            extra_link_args = []
+        exts.append( Extension('binascii', ['binascii.c'],
+                               extra_compile_args = extra_compile_args,
+                               libraries = libraries,
+                               extra_link_args = extra_link_args) )
+
         # Gustavo Niemeyer's bz2 module.
         if (self.compiler.find_library_file(lib_dirs, 'bz2')):
             if sys.platform == "darwin":