Added stripped-down internal copy of boost.

Added stripped-down internal copy of boost.

If CMake does not detect a recent boost installation, the internal copy
is used.  If this is the case, the boost headers will also be installed
to PREFIX/include/gromacs/external/boost/ to make it possible to
reference them from installed headers (exception implementation will
need this).

Some TODO items remain on the CMake side to make things work smoothly
for users when the internal boost is used.

Minimal boost and initial CMake support by Roland Schulz, installation
of boost headers by Teemu Murtola.