Build option --with-python should default to yes

When building BIND on a minimal build system (where there's no python or python-ply), the "dnssec-keymgr" utility does not get built or installed. However, this tool is rather important. I wasn't even aware of it, and it took a long while to figure out why it wasn't being built.

I feel that newer versions of BIND should default to yes for --with-python. This will ensure that dnssec-keymgr gets built and installed by default. This should be fine on most modern Linux distributions because they should meet the minimum requirements.

There are a small number of common distributions out there, such as CentOS 6, which ship with Python 2.6. When building for them, one can use --without-python.

By the way, I discussed this with Mark Andrews at RIPE 77, and he agreed with my suggestion.