Set default target to "lib" instead of "libdiffpy".

lib is an alias target, which generates all header files
together with libdiffpy.so.
Install headers only after libdiffpy.so compiles.
Work around serialization headers bug in boost 1.56.

serialization/shared_ptr.hpp uses singleton, but does not include its
declaration.  singleton is included from serialization/export.hpp so
if it is included before shared_ptr.hpp, the bug does not bite.
Make features.hpp depend on the has_objcryst flag.

Fix incorrect dependency for version.hpp.