Support build from swig in install.py

[earlchew]

As a developer, I want to use a single step build process, so that I can install directly from the swig input file.

$ python ./setup.py install
running install
running build_ext
building 'libnfs._libnfs' extension
swigging libnfs/libnfs.i to libnfs/libnfs_wrap.c
swig -python -shadow -threads -o libnfs/libnfs_wrap.c libnfs/libnfs.i
creating build
creating build/temp.linux-x86_64-2.7
creating build/temp.linux-x86_64-2.7/libnfs
gcc -pthread -fno-strict-aliasing -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -fPIC -I/usr/include/python2.7 -c libnfs/libnfs_wrap.c -o build/temp.linux-x86_64-2.7/libnfs/libnfs_wrap.o -g
...