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With patch for the zlib module (issue20193) which uses the "unspecified" default value:
$ ./python -m pydoc zlib
Traceback (most recent call last):
 File "/home/serhiy/py/cpython/Lib/inspect.py", line 1997, in wrap_value
 value = eval(s, module_dict)
 File "<string>", line 1, in <module>
NameError: name 'unspecified' is not defined
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
 File "/home/serhiy/py/cpython/Lib/inspect.py", line 2000, in wrap_value
 value = eval(s, sys_module_dict)
 File "<string>", line 1, in <module>
NameError: name 'unspecified' is not defined
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
 File "/home/serhiy/py/cpython/Lib/runpy.py", line 189, in _run_module_as_main
 "__main__", mod_spec)
 File "/home/serhiy/py/cpython/Lib/runpy.py", line 87, in _run_code
 exec(code, run_globals)
 File "/home/serhiy/py/cpython/Lib/pydoc.py", line 2593, in <module>
 cli()
 File "/home/serhiy/py/cpython/Lib/pydoc.py", line 2558, in cli
 help.help(arg)
 File "/home/serhiy/py/cpython/Lib/pydoc.py", line 1840, in help
 elif request: doc(request, 'Help on %s:', output=self._output)
 File "/home/serhiy/py/cpython/Lib/pydoc.py", line 1578, in doc
 pager(render_doc(thing, title, forceload))
 File "/home/serhiy/py/cpython/Lib/pydoc.py", line 1571, in render_doc
 return title % desc + '\n\n' + renderer.document(object, name)
 File "/home/serhiy/py/cpython/Lib/pydoc.py", line 356, in document
 if inspect.ismodule(object): return self.docmodule(*args)
 File "/home/serhiy/py/cpython/Lib/pydoc.py", line 1142, in docmodule
 contents.append(self.document(value, key, name))
 File "/home/serhiy/py/cpython/Lib/pydoc.py", line 358, in document
 if inspect.isroutine(object): return self.docroutine(*args)
 File "/home/serhiy/py/cpython/Lib/pydoc.py", line 1323, in docroutine
 signature = inspect.signature(object)
 File "/home/serhiy/py/cpython/Lib/inspect.py", line 1468, in signature
 return Signature.from_builtin(obj)
 File "/home/serhiy/py/cpython/Lib/inspect.py", line 2052, in from_builtin
 p(name, default)
 File "/home/serhiy/py/cpython/Lib/inspect.py", line 2038, in p
 default_node = RewriteSymbolics().visit(default_node)
 File "/home/serhiy/py/cpython/Lib/ast.py", line 245, in visit
 return visitor(node)
 File "/home/serhiy/py/cpython/Lib/inspect.py", line 2030, in visit_Name
 return wrap_value(node.id)
 File "/home/serhiy/py/cpython/Lib/inspect.py", line 2002, in wrap_value
 raise RuntimeError()
RuntimeError

Just checking--that happens with current trunk? Revision 32f9e0ae23f7 or newer?

That happens with current trunk (revision 1469c4fde8cd) and a patch for zlib from issue20193 [1].
[1] http://bugs.python.org/file33523/zlib_clinic.patch 

I just built from a new clone of trunk, and it works fine for me. I didn't see the exception you report--I get the documentation for zlib.
Do you still get the error if you build from a new clone of the current trunk?

> I just built from a new clone of trunk, and it works fine for me.
It works fine for me, too (revision 8f11493cf727).

Closing. Reopen it if you get a reproducible test case with a fresh checkout of trunk.

I just built from a new clone of trunk (last revision is d0e2437136f5), and I still get the error.

Seems this is related issue:
>>> import zlib, inspect
>>> zlib.compressobj.__text_signature__
'(level=Z_DEFAULT_COMPRESSION, method=DEFLATED, wbits=MAX_WBITS, memLevel=DEF_MEM_LEVEL, strategy=Z_DEFAULT_STRATEGY, zdict=None)'
>>> str(inspect.signature(zlib.compressobj))
'(level=-1, method=8, wbits=15, memLevel=8, strategy=0, zdict=None)'
help(zlib.compress) shows the '(bytes, level=-1)' signature.
inspect.signature() evaluates default values and instead of expected named constant it outputs numeric value which is often implementation detail.

Yes, that is how it must work. This is symmetric with pure Python functions:
 >>> import inspect
 >>> import zlib
 >>> def foo(a=zlib.Z_DEFAULT_COMPRESSION): pass
 ... 
 >>> str(inspect.signature(foo))
 '(a=-1)'

I missed the detail that you were working with a patch applied.
I suspect the problem is, you misspelled one of the expressions you provided as a default value. Try str(inspect.signature(x)) on every function you converted. I bet one of them will throw an exception.
Yes this is error-prone.

> Yes, that is how it must work. This is symmetric with pure Python 
functions:
This is because in pure Python functions we have no enough information. I 
believe than origin expression is more useful in the help. Compare:
>>> import re, inspect
>>> p = re.compile('')
>>> p.match.__text_signature__
'(string, pos=0, endpos=sys.maxsize)'
>>> str(inspect.signature(p.match))
'(string, pos=0, endpos=2147483647)'
> I suspect the problem is, you misspelled one of the expressions you provided
> as a default value.
>>> import zlib, inspect
>>> zlib.decompress.__text_signature__
'(data, wbits=unspecified, bufsize=unspecified)'
>>> str(inspect.signature(zlib.decompress))
Traceback (most recent call last):
 File "/home/serhiy/py/cpython/Lib/inspect.py", line 1997, in wrap_value
 value = eval(s, module_dict)
 File "<string>", line 1, in <module>
NameError: name 'unspecified' is not defined
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
 File "/home/serhiy/py/cpython/Lib/inspect.py", line 2000, in wrap_value
 value = eval(s, sys_module_dict)
 File "<string>", line 1, in <module>
NameError: name 'unspecified' is not defined
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/home/serhiy/py/cpython/Lib/inspect.py", line 1468, in signature
 return Signature.from_builtin(obj)
 File "/home/serhiy/py/cpython/Lib/inspect.py", line 2052, in from_builtin
 p(name, default)
 File "/home/serhiy/py/cpython/Lib/inspect.py", line 2038, in p
 default_node = RewriteSymbolics().visit(default_node)
 File "/home/serhiy/py/cpython/Lib/ast.py", line 245, in visit
 return visitor(node)
 File "/home/serhiy/py/cpython/Lib/inspect.py", line 2030, in visit_Name
 return wrap_value(node.id)
 File "/home/serhiy/py/cpython/Lib/inspect.py", line 2002, in wrap_value
 raise RuntimeError()
RuntimeError

> This is because in pure Python functions we have no enough
> information. I believe than origin expression is more useful
> in the help.
That doesn't matter. inspect.Signature would have to change in order to provide the original expression, and it's not going to for 3.4.
In zlib.decompress, remove the "= unspecified". You can have a c_default without a default value now.

Without the "= unspecified" parameters will be non-optional.

Since Argument Clinic no more supports "unspecified" default value, this issue can be closed.