Assorted autotools fixups

This drops the checked-in auto-generated autotools files in favor of an autogen.sh shell script that creates them for the local system.  Rationale, from the commit log.

```
Don't version generated files, have autogen.sh create them at build
time. This should make life easier for people that have to re-run
autoconf (or automake) for their system, then end up with a git
index full of clobbered autotools boilerplate.

When I say 'people', I mean 'me.'
```

The last commit makes configure check for and link against the system libgtest. I have that one installed so it's kind of a drag to have to download the tarball (or is it a zip file?) manually (and also because I keep blowing it away accidentally with `git clean -dfx`.)

Feedback welcome.