[Announcement] STP Shared library now built by default

[Delcypher]

Hi,

Just a small announcement. ``libstp`` can now be built as a shared
library in upstream STP and it is now the default (see
BUILD_SHARED_LIBS CMake option).

To maintain backwards compatibility, a static version of ``libstp`` is
also built by default (see ALSO_BUILD_STATIC_LIB CMake option) but
note that the ``stp`` executable will be linked against the shared
library in this case.

Thanks,
Dan.