This has been reported downstream:
http://bugs.gentoo.org/show_bug.cgi?id=246745

More information about this can be seen at:
http://www.gentoo.org/proj/en/qa/asneeded.xml

The error can be reproduced when compiling with the following LDFLAGS:
LDFLAGS="${LDFLAGS} -Wl,-z,-defs -Wl,--no-undefined"

And is:

libxml2-py.c:(.text+0x10969): undefined reference to `PyArg_ParseTuple'
libxml2-py.c:(.text+0x1097f): undefined reference to `_Py_NoneStruct'
.libs/libxml2-py.o: In function `libxml_xmlXPathNormalizeFunction':
libxml2-py.c:(.text+0x109bc): undefined reference to `PyArg_ParseTuple'
libxml2-py.c:(.text+0x109cb): undefined reference to `_Py_NoneStruct'
.libs/libxml2-py.o: In function `libxml_xmlFreeNs':
libxml2-py.c:(.text+0x10a09): undefined reference to `PyArg_ParseTuple'
libxml2-py.c:(.text+0x10a19): undefined reference to `_Py_NoneStruct'
.libs/libxml2-py.o: In function `libxml_xmlValidCtxtNormalizeAttributeValue':
libxml2-py.c:(.text+0x10a6e): undefined reference to `PyArg_ParseTuple'
libxml2-py.c:(.text+0x10a82): undefined reference to `_Py_NoneStruct'
.libs/libxml2-py.o: In function `libxml_xmlSchemaSetValidOptions':
libxml2-py.c:(.text+0x10ae9): undefined reference to `PyArg_ParseTuple'
libxml2-py.c:(.text+0x10afe): undefined reference to `_Py_NoneStruct'
.libs/libxml2-py.o: In function `libxml_xmlCheckLanguageID':
libxml2-py.c:(.text+0x10b36): undefined reference to `PyArg_ParseTuple'
.libs/libxml2-py.o: In function `libxml_xmlRelaxNGFreeParserCtxt':
libxml2-py.c:(.text+0x10b79): undefined reference to `PyArg_ParseTuple'
libxml2-py.c:(.text+0x10b89): undefined reference to `_Py_NoneStruct'
.libs/libxml2-py.o: In function `libxml_xmlXPathInit':
libxml2-py.c:(.text+0x10bbc): undefined reference to `_Py_NoneStruct'
.libs/libxml2-py.o: In function `libxml_xmlUCSIsMalayalam':
libxml2-py.c:(.text+0x10be6): undefined reference to `PyArg_ParseTuple'
.libs/libxml2-py.o: In function `libxml_xmlGetID':
libxml2-py.c:(.text+0x10c29): undefined reference to `PyArg_ParseTuple'
libxml2-py.c:(.text+0x10c3f): undefined reference to `_Py_NoneStruct'
.libs/libxml2-py.o: In function `libxml_xmlValidateOneElement':
libxml2-py.c:(.text+0x10c80): undefined reference to `PyArg_ParseTuple'
libxml2-py.c:(.text+0x10c94): undefined reference to `_Py_NoneStruct'
.libs/libxml2-py.o: In function `libxml_xmlNanoFTPCleanup':
libxml2-py.c:(.text+0x10cec): undefined reference to `_Py_NoneStruct'
.libs/libxml2-py.o: In function `libxml_xmlSchemaWhiteSpaceReplace':
libxml2-py.c:(.text+0x10d16): undefined reference to `PyArg_ParseTuple'
.libs/libxml2-py.o: In function `libxml_xmlUCSIsHangulJamo':
libxml2-py.c:(.text+0x10d56): undefined reference to `PyArg_ParseTuple'
.libs/libxml2-py.o: In function `libxml_xmlDebugDumpNodeList':
libxml2-py.c:(.text+0x10da3): undefined reference to `PyArg_ParseTuple'
libxml2-py.c:(.text+0x10db3): undefined reference to `_Py_NoneStruct'
libxml2-py.c:(.text+0x10dc5): undefined reference to `PyFile_Type'
libxml2-py.c:(.text+0x10dcf): undefined reference to `PyType_IsSubtype'
libxml2-py.c:(.text+0x10de0): undefined reference to `PyFile_AsFile'
.libs/libxml2-py.o: In function `libxml_xmlUCSIsMathematicalAlphanumericSymbols':
libxml2-py.c:(.text+0x10e36): undefined reference to `PyArg_ParseTuple'
.libs/libxml2-py.o: In function `libxml_xmlFreeParserInputBuffer':
libxml2-py.c:(.text+0x10e79): undefined reference to `PyArg_ParseTuple'
libxml2-py.c:(.text+0x10e89): undefined reference to `_Py_NoneStruct'
.libs/libxml2-py.o: In function `libxml_xmlTextReaderExpand':
libxml2-py.c:(.text+0x10ec6): undefined reference to `PyArg_ParseTuple'
libxml2-py.c:(.text+0x10edc): undefined reference to `_Py_NoneStruct'
.libs/libxml2-py.o: In function `libxml_xmlReaderForFile':
libxml2-py.c:(.text+0x10f20): undefined reference to `PyArg_ParseTuple'
.libs/libxml2-py.o: In function `libxml_xmlStrstr':
libxml2-py.c:(.text+0x10f69): undefined reference to `PyArg_ParseTuple'
.libs/libxml2-py.o: In function `libxml_xmlXPathNsLookup':
libxml2-py.c:(.text+0x10fa9): undefined reference to `PyArg_ParseTuple'
libxml2-py.c:(.text+0x10fbf): undefined reference to `_Py_NoneStruct'
.libs/libxml2-py.o: In function `libxml_xmlUCSIsBopomofo':
libxml2-py.c:(.text+0x10ff6): undefined reference to `PyArg_ParseTuple'
.libs/libxml2-py.o: In function `libxml_xmlGetDocEntity':
libxml2-py.c:(.text+0x11039): undefined reference to `PyArg_ParseTuple'
libxml2-py.c:(.text+0x1104f): undefined reference to `_Py_NoneStruct'
collect2: ld returned 1 exit status
make[1]: *** [libxml2mod.la] Error 1
make[1]: Leaving directory `/var/tmp/portage/dev-libs/libxml2-2.7.7/work/libxml2-2.7.7/python-2.6'
make: *** [all-recursive] Error 1

I attach full build.log

Christian Ruppert suggested that this could be fixed running:
sed -i -r -e 's:(@WITH_PYTHON_TRUE@libxml2mod_la_LIBADD =.*):\1 -lpython$(PYTHON_VERSION):' python/Makefile.in