title: i have an error when executing "from lxml import etree" in the python command line after successfully installed lxml by pip
body: <pre>
      bash-3.2$ pip install lxml-2.3.5.tgz 
      Unpacking ./lxml-2.3.5.tgz
        Running setup.py egg_info for package from file:///Users/apple/workspace/pythonhome/misc/lxml-2.3.5.tgz
          Building lxml version 2.3.5.
          Building with Cython 0.17.
          Using build configuration of libxslt 1.1.27
          Building against libxml2/libxslt in the following directory: /usr/local/lib
  
          warning: no previously-included files found matching '*.py'
      Installing collected packages: lxml
        Running setup.py install for lxml
          Building lxml version 2.3.5.
          Building with Cython 0.17.
          Using build configuration of libxslt 1.1.27
          Building against libxml2/libxslt in the following directory: /usr/local/lib
          skipping 'src/lxml/lxml.etree.c' Cython extension (up-to-date)
          building 'lxml.etree' extension
          llvm-gcc-4.2 -fno-strict-aliasing -fno-common -dynamic -arch i386 -arch x86_64 -g -O2 -DNDEBUG -g -O3 -I/usr/local/include -I/usr/local/include/libxml2 -I/Library/Frameworks/Python.framework/Versions/2.7/include/python2.7 -c src/lxml/lxml.etree.c -o build/temp.macosx-10.6-intel-2.7/src/lxml/lxml.etree.o -w -flat_namespace
          llvm-gcc-4.2 -bundle -undefined dynamic_lookup -arch i386 -arch x86_64 -g build/temp.macosx-10.6-intel-2.7/src/lxml/lxml.etree.o -L/usr/local/lib -lxslt -lexslt -lxml2 -lz -lm -o build/lib.macosx-10.6-intel-2.7/lxml/etree.so
          skipping 'src/lxml/lxml.objectify.c' Cython extension (up-to-date)
          building 'lxml.objectify' extension
          llvm-gcc-4.2 -fno-strict-aliasing -fno-common -dynamic -arch i386 -arch x86_64 -g -O2 -DNDEBUG -g -O3 -I/usr/local/include -I/usr/local/include/libxml2 -I/Library/Frameworks/Python.framework/Versions/2.7/include/python2.7 -c src/lxml/lxml.objectify.c -o build/temp.macosx-10.6-intel-2.7/src/lxml/lxml.objectify.o -w -flat_namespace
          llvm-gcc-4.2 -bundle -undefined dynamic_lookup -arch i386 -arch x86_64 -g build/temp.macosx-10.6-intel-2.7/src/lxml/lxml.objectify.o -L/usr/local/lib -lxslt -lexslt -lxml2 -lz -lm -o build/lib.macosx-10.6-intel-2.7/lxml/objectify.so
  
      Successfully installed lxml
      Cleaning up...
      bash-3.2$ python
      Python 2.7.3 (v2.7.3:70274d53c1dd, Apr  9 2012, 20:52:43) 
      [GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
      Type "help", "copyright", "credits" or "license" for more information.
      >>> import lxml
      >>> import html
      Traceback (most recent call last):
        File "", line 1, in 
      ImportError: No module named html
      >>> from lxml import html
      Traceback (most recent call last):
        File "", line 1, in 
        File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/lxml/html/__init__.py", line 12, in 
          from lxml import etree
      ImportError: dlopen(/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/lxml/etree.so, 2): Symbol not found: _exsltDateXpathCtxtRegister
        Referenced from: /Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/lxml/etree.so
        Expected in: flat namespace
       in /Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/lxml/etree.so
  
  </pre>
  
  <p>get same error again for older version lxml2.3.4</p>
  
  <pre>
  
      bash-3.2$ pip install lxml-2.3.4.tgz 
      Unpacking ./lxml-2.3.4.tgz
        Running setup.py egg_info for package from file:///Users/apple/workspace/pythonhome/misc/lxml-2.3.4.tgz
          Building lxml version 2.3.4.
          Building with Cython 0.17.
          Using build configuration of libxslt 1.1.27
          Building against libxml2/libxslt in the following directory: /usr/local/lib
  
          warning: no previously-included files found matching '*.py'
      Installing collected packages: lxml
        Running setup.py install for lxml
          Building lxml version 2.3.4.
          Building with Cython 0.17.
          Using build configuration of libxslt 1.1.27
          Building against libxml2/libxslt in the following directory: /usr/local/lib
          skipping 'src/lxml/lxml.etree.c' Cython extension (up-to-date)
          building 'lxml.etree' extension
          llvm-gcc-4.2 -fno-strict-aliasing -fno-common -dynamic -arch i386 -arch x86_64 -g -O2 -DNDEBUG -g -O3 -I/usr/local/include -I/usr/local/include/libxml2 -I/Library/Frameworks/Python.framework/Versions/2.7/include/python2.7 -c src/lxml/lxml.etree.c -o build/temp.macosx-10.6-intel-2.7/src/lxml/lxml.etree.o -w -flat_namespace
          llvm-gcc-4.2 -bundle -undefined dynamic_lookup -arch i386 -arch x86_64 -g build/temp.macosx-10.6-intel-2.7/src/lxml/lxml.etree.o -L/usr/local/lib -lxslt -lexslt -lxml2 -lz -lm -o build/lib.macosx-10.6-intel-2.7/lxml/etree.so
          skipping 'src/lxml/lxml.objectify.c' Cython extension (up-to-date)
          building 'lxml.objectify' extension
          llvm-gcc-4.2 -fno-strict-aliasing -fno-common -dynamic -arch i386 -arch x86_64 -g -O2 -DNDEBUG -g -O3 -I/usr/local/include -I/usr/local/include/libxml2 -I/Library/Frameworks/Python.framework/Versions/2.7/include/python2.7 -c src/lxml/lxml.objectify.c -o build/temp.macosx-10.6-intel-2.7/src/lxml/lxml.objectify.o -w -flat_namespace
          llvm-gcc-4.2 -bundle -undefined dynamic_lookup -arch i386 -arch x86_64 -g build/temp.macosx-10.6-intel-2.7/src/lxml/lxml.objectify.o -L/usr/local/lib -lxslt -lexslt -lxml2 -lz -lm -o build/lib.macosx-10.6-intel-2.7/lxml/objectify.so
  
      Successfully installed lxml
      Cleaning up...
      bash-3.2$ python
      Python 2.7.3 (v2.7.3:70274d53c1dd, Apr  9 2012, 20:52:43) 
      [GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
      Type "help", "copyright", "credits" or "license" for more information.
      >>> import os
      >>> import lxml
      >>> from lxml import etree
      Traceback (most recent call last):
        File "", line 1, in 
      ImportError: dlopen(/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/lxml/etree.so, 2): Symbol not found: _exsltDateXpathCtxtRegister
        Referenced from: /Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/lxml/etree.so
        Expected in: flat namespace
       in /Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/lxml/etree.so
  
  </pre>