precompiled headers for boost python

[springmeyer]

I think we should give a shot to precompiling the boost python headers to speed up python-mapnik compiles. In the past I've tried this for mapnik core (precompiling spirit) but did not see a major benefit (that outweighed the hassle of setup) - but I think we should revisit trying to pre-compile boost/python.hpp.

[DeoLeung]

hi, I'm now using python3.6 and ran into the issue about boost_python, I tried

cd /tmp/boost_1_55_0/ && \
    ./bootstrap.sh --with-python=/usr/local/bin/python3 --with-python-version=3.6 --with-python-root=/usr/local/lib/python3.6 && \
    # this may fail (failed updating xxx) need to ignore or install 3rd party packages
    ./b2 --enable-unicode=ucs4 install || echo 'finished'

but after that I can't find the boost_python, how can I link that?