building convolve breaks on Windows with MSVC

This is the output I get when I try to run setup.py build on Windows using MSVC as the compiler:

```
cythoning astropy\nddata\convolution\boundary_extend.pyx to astropy\nddata\convo
lution\boundary_extend.c
building 'astropy.nddata.convolution.boundary_extend' extension
creating build\temp.win32-2.7\Release\astropy\nddata
creating build\temp.win32-2.7\Release\astropy\nddata\convolution
C:\Program Files\Microsoft Visual Studio 9.0\VC\BIN\cl.exe /c /nologo /Ox /MD /W3 /GS- /DNDEBUG -IC:\Python27\lib\site-packages\numpy-1.6.1-py2.7-win32.egg\numpy\core\include -IC:\Python27\include -IC:\Python27\PC /Tcastropy\nddata\convolution\boundary_extend.c /Fobuild\temp.win32-2.7\Release\astropy\nddata\convolution\boundary_extend.obj
boundary_extend.c
astropy\nddata\convolution\boundary_extend.c(1368) : warning C4018: '<' : signed/unsigned mismatch
astropy\nddata\convolution\boundary_extend.c(1379) : warning C4013: 'isnan' undefined; assuming extern returning int
astropy\nddata\convolution\boundary_extend.c(1555) : warning C4018: '<' : signed/unsigned mismatch
astropy\nddata\convolution\boundary_extend.c(2193) : warning C4018: '<' : signed/unsigned mismatch
astropy\nddata\convolution\boundary_extend.c(2204) : warning C4018: '<' : signed/unsigned mismatch
astropy\nddata\convolution\boundary_extend.c(2439) : warning C4018: '<' : signed/unsigned mismatch
astropy\nddata\convolution\boundary_extend.c(2450) : warning C4018: '<' : signed/unsigned mismatch
astropy\nddata\convolution\boundary_extend.c(3217) : warning C4018: '<' : signed/unsigned mismatch
astropy\nddata\convolution\boundary_extend.c(3228) : warning C4018: '<' : signed/unsigned mismatch
astropy\nddata\convolution\boundary_extend.c(3239) : warning C4018: '<' : signed/unsigned mismatch
astropy\nddata\convolution\boundary_extend.c(3522) : warning C4018: '<' : signed/unsigned mismatch
astropy\nddata\convolution\boundary_extend.c(3533) : warning C4018: '<' : signed/unsigned mismatch
astropy\nddata\convolution\boundary_extend.c(3544) : warning C4018: '<' : signed/unsigned mismatch
C:\Program Files\Microsoft Visual Studio 9.0\VC\BIN\link.exe /DLL /nologo /INCREMENTAL:NO /LIBPATH:C:\Python27\libs /LIBPATH:C:\Python27\PCbuild /EXPORT:initboundary_extend build\temp.win32-2.7\Release\astropy\nddata\convolution\boundary_extend.obj /OUT:build\lib.win32-2.7\astropy\nddata\convolution\boundary_extend.pyd /IMPLIB:build\temp.win32-2.7\Release\astropy\nddata\convolution\boundary_extend.lib /MANIFESTFILE:build\temp.win32-2.7\Release\astropy\nddata\convolution\boundary_extend.pyd.manifest /MANIFEST
   Creating library build\temp.win32-2.7\Release\astropy\nddata\convolution\boundary_extend.lib and object build\temp.win32-2.7\Release\astropy\nddata\convolution\boundary_extend.exp
boundary_extend.obj : error LNK2019: unresolved external symbol _isnan referenced in function ___pyx_pf_7astropy_6nddata_11convolution_15boundary_extend_convolve1d_boundary_extend
build\lib.win32-2.7\astropy\nddata\convolution\boundary_extend.pyd : fatal error
 LNK1120: 1 unresolved externals
error: command '"C:\Program Files\Microsoft Visual Studio 9.0\VC\BIN\link.exe"'
failed with exit status 1120
```

I'm not sure.  Could this be a Cython bug?  For what it's worth, the Cython on this Python installation was built with MSVC, and the build succeeds no problem when I compile in mingw.