Switching XPath node sorting to Timsort

I use libxml xpath engine on quite large (and mostly "flat") xml files.
It seems that Shellsort, that is used in xmlXPathNodeSetSort is a
performance bottleneck for my case. I have read some posts about sorting
in libxml in the libxml archive, but I agree that qsort was not the way
to go. I experimented with Timsort instead and my results were good for
me. For about 10000 nodes, my test was about 5x faster with Timsort,
for 1000 nodes about 10% faster, for small data files, the difference
was not measurable.
* timsort.h: the algorithm, kept in a separate header
* xpath.c: plug in the new algorithm in xmlXPathNodeSetSort
* Makefile.am: add the header to the EXTRA_DIST
* doc/apibuild.py: avoid indexing the new header