conda: get Boost library with Python version

[bkpoon]

This adds the Python version number, if necessary, to the Boost.Python library. This enables the linking of Boost libraries from conda packages. However, linking only works with compilers that are ABI compatible with the ones used to build the conda packages.

[Anthchirp]

This sounds interesting - and timely given that I found out today that we can't use the boost libraries from conda (dials/dials@c37a4c9).

How far away do you think we are from making that happen?

Otherwise - I will run full checks post-merge.

[bkpoon]

I saw your changes to the DIALS environments, so I figured now would be a good time to merge this branch. The main problem with using Boost from conda-forge is an ABI issue with GCC (https://gcc.gnu.org/onlinedocs/libstdc++/manual/using_dual_abi.html). I have boost and boost-cpp built with the old ABI in the cctbx-old-abi channel. Those packages will work with CentOS 6, CentOS 7, and Ubuntu 14.04. However, those packages probably won't link correctly with msgpack-c from conda-forge. However, if you only care about the headers from msgpack-c, then it doesn't matter (which is why the current CCTBX environments have msgpack-c, but no boost-cpp), but you might get messages from conda about inconsistent environments.

I have CCTBX building with the boost packages, but this change is only necessary if code is being linked to Boost.Python libraries.

[bkpoon]

Also, the newpy branch in cctbx_project is not necessary for you to build with the conda packages of Boost. That branch basically has updated environments and enables tests for Python 3.7 and 3.8.

As long as you do not have a boost directory in modules, and the boost and boost-cpp packages are in your conda environment, the build should work. It's safest to delete your existing build directory in case of leftover Boost objects/libraries.

[phyy-nx]

Similar to what we did for psana1, I believe. Looks good to me.