Remove library dependencies from wrapping targets.

Previously, the swig source code generation targets depended on the libraries
being built. This slows down the build time, and is not actually correct: the
swig targets only depend on header files, not on compiled libraries.

So I removed these dependencies. The issue is then that if you edit a header
file, CMake won't know to re-run the SWIG targets. In this case, delete the
SWIG generated source code, and the swig target will re-run.