Create a shared library

If BUILD_SHARED_LIBS was falsy (default), we were only
generating a static library.  In the future, b will be used
by other projects (possibly in other languages), and dynamic
linking is easier in such environments.  Create a shared
library (CMake target 'b_shared').

This requires -fPIC on Linux (ELF x86_64).
Support multiple targets passed to CMake

This allows e.g.

    $ make TestProcess TestQuestionQueue

which will build both TestProcess and TestQuestionQueue.
WIP Python