Add autoconf macros for SWIG.

The AX_SWIG_PYTHON macro depends on the AX_PKG_SWIG and AX_PYTHON_DEVEL
macros.
Fix VPATH builds.
Switch to non-recursive make.
Create Python extension using SWIG.
Add build directory to .gitignore
Trim and comment configure.ac.
Rename addr_example to c_adder.
Test the C client of libadder.