Fail to configure without an LLVM-friendly Python

Fail to configure without an LLVM-friendly Python

This addresses issue #2720. According to LLVM's documentation, it requires a
version of Python between 2.4 and 2.7. Without the proper version, LLVM fails
to build with cryptic errors. Prior to this commit, the configure script
checked for the `python` command in the environment, but didn't actually check
the version, which can cause problems e.g. on Linux distros where the default
is Python 3. Now the configure script always prefers to select a more specific
version of Python when available, in the order `python2.7` > `python2.6` >
`python2` > `python`, and will always check to ensure that the interpreter's
version is in the correct range.