test_wcsapi_extension failing on MacOS X

@mdboom - I'm getting this error on MacOS X (10.8 and 10.9, two different machines):

```
=================================== FAILURES ===================================
____________________________ test_wcsapi_extension _____________________________

tmpdir = local('/private/var/folders/zy/t1l3sx310d3d6p0kyxqzlrnr0000gr/T/pytest-5/test_wcsapi_extension0')

    def test_wcsapi_extension(tmpdir):
        # Test that we can build a simple C extension with the astropy.wcs C API
    
        setup_path = os.path.join(os.path.dirname(__file__), 'setup.py')
    
        # Build the extension
        subprocess.check_call([
            sys.executable, setup_path,
>           'install', '--install-lib={0}'.format(tmpdir)])

astropy/wcs/tests/extension/test_extension.py:18: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

popenargs = (['/opt/local/Library/Frameworks/Python.framework/Versions/2.7/Resources/Python.app/Contents/MacOS/Python', '/private/... 'install', '--install-lib=/private/var/folders/zy/t1l3sx310d3d6p0kyxqzlrnr0000gr/T/pytest-5/test_wcsapi_extension0'],)
kwargs = {}, retcode = 1
cmd = ['/opt/local/Library/Frameworks/Python.framework/Versions/2.7/Resources/Python.app/Contents/MacOS/Python', '/private/v...', 'install', '--install-lib=/private/var/folders/zy/t1l3sx310d3d6p0kyxqzlrnr0000gr/T/pytest-5/test_wcsapi_extension0']

    def check_call(*popenargs, **kwargs):
        """Run command with arguments.  Wait for command to complete.  If
        the exit code was zero then return, otherwise raise
        CalledProcessError.  The CalledProcessError object will have the
        return code in the returncode attribute.
    
        The arguments are the same as for the Popen constructor.  Example:
    
        check_call(["ls", "-l"])
        """
        retcode = call(*popenargs, **kwargs)
        if retcode:
            cmd = kwargs.get("args")
            if cmd is None:
                cmd = popenargs[0]
>           raise CalledProcessError(retcode, cmd)
E           CalledProcessError: Command '['/opt/local/Library/Frameworks/Python.framework/Versions/2.7/Resources/Python.app/Contents/MacOS/Python', u'/private/var/folders/zy/t1l3sx310d3d6p0kyxqzlrnr0000gr/T/astropy-test-kCY8AI/lib.macosx-10.8-x86_64-2.7/astropy/wcs/tests/extension/setup.py', u'install', u'--install-lib=/private/var/folders/zy/t1l3sx310d3d6p0kyxqzlrnr0000gr/T/pytest-5/test_wcsapi_extension0']' returned non-zero exit status 1

/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/subprocess.py:540: CalledProcessError
------------------------------- Captured stdout --------------------------------
running install
running build
running build_ext
building 'wcsapi_test' extension
creating build
creating build/temp.macosx-10.8-x86_64-2.7
creating build/temp.macosx-10.8-x86_64-2.7/private
creating build/temp.macosx-10.8-x86_64-2.7/private/var
creating build/temp.macosx-10.8-x86_64-2.7/private/var/folders
creating build/temp.macosx-10.8-x86_64-2.7/private/var/folders/zy
creating build/temp.macosx-10.8-x86_64-2.7/private/var/folders/zy/t1l3sx310d3d6p0kyxqzlrnr0000gr
creating build/temp.macosx-10.8-x86_64-2.7/private/var/folders/zy/t1l3sx310d3d6p0kyxqzlrnr0000gr/T
creating build/temp.macosx-10.8-x86_64-2.7/private/var/folders/zy/t1l3sx310d3d6p0kyxqzlrnr0000gr/T/astropy-test-kCY8AI
creating build/temp.macosx-10.8-x86_64-2.7/private/var/folders/zy/t1l3sx310d3d6p0kyxqzlrnr0000gr/T/astropy-test-kCY8AI/lib.macosx-10.8-x86_64-2.7
creating build/temp.macosx-10.8-x86_64-2.7/private/var/folders/zy/t1l3sx310d3d6p0kyxqzlrnr0000gr/T/astropy-test-kCY8AI/lib.macosx-10.8-x86_64-2.7/astropy
creating build/temp.macosx-10.8-x86_64-2.7/private/var/folders/zy/t1l3sx310d3d6p0kyxqzlrnr0000gr/T/astropy-test-kCY8AI/lib.macosx-10.8-x86_64-2.7/astropy/wcs
creating build/temp.macosx-10.8-x86_64-2.7/private/var/folders/zy/t1l3sx310d3d6p0kyxqzlrnr0000gr/T/astropy-test-kCY8AI/lib.macosx-10.8-x86_64-2.7/astropy/wcs/tests
creating build/temp.macosx-10.8-x86_64-2.7/private/var/folders/zy/t1l3sx310d3d6p0kyxqzlrnr0000gr/T/astropy-test-kCY8AI/lib.macosx-10.8-x86_64-2.7/astropy/wcs/tests/extension
/Users/tom/usr/bin/clang -fno-strict-aliasing -fno-common -dynamic -pipe -Os -fwrapv -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -I/private/var/folders/zy/t1l3sx310d3d6p0kyxqzlrnr0000gr/T/astropy-test-kCY8AI/lib.macosx-10.8-x86_64-2.7/astropy/wcs/include/astropy_wcs -I/private/var/folders/zy/t1l3sx310d3d6p0kyxqzlrnr0000gr/T/astropy-test-kCY8AI/lib.macosx-10.8-x86_64-2.7/astropy/wcs/include/wcslib -I/opt/local/Library/Frameworks/Python.framework/Versions/2.7/include/python2.7 -c /private/var/folders/zy/t1l3sx310d3d6p0kyxqzlrnr0000gr/T/astropy-test-kCY8AI/lib.macosx-10.8-x86_64-2.7/astropy/wcs/tests/extension/wcsapi_test.c -o build/temp.macosx-10.8-x86_64-2.7/private/var/folders/zy/t1l3sx310d3d6p0kyxqzlrnr0000gr/T/astropy-test-kCY8AI/lib.macosx-10.8-x86_64-2.7/astropy/wcs/tests/extension/wcsapi_test.o
------------------------------- Captured stderr --------------------------------
In file included from /private/var/folders/zy/t1l3sx310d3d6p0kyxqzlrnr0000gr/T/astropy-test-kCY8AI/lib.macosx-10.8-x86_64-2.7/astropy/wcs/tests/extension/wcsapi_test.c:2:
In file included from /private/var/folders/zy/t1l3sx310d3d6p0kyxqzlrnr0000gr/T/astropy-test-kCY8AI/lib.macosx-10.8-x86_64-2.7/astropy/wcs/include/astropy_wcs/astropy_wcs_api.h:5:
/private/var/folders/zy/t1l3sx310d3d6p0kyxqzlrnr0000gr/T/astropy-test-kCY8AI/lib.macosx-10.8-x86_64-2.7/astropy/wcs/include/astropy_wcs/pyutil.h:15:10: fatal error: 'numpy/arrayobject.h' file not found
#include <numpy/arrayobject.h>
         ^
1 error generated.
error: command '/Users/tom/usr/bin/clang' failed with exit status 1
=============== 1 failed, 268 passed, 1 xfailed in 3.44 seconds ================
```